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Planning Photography of Microfossils iii

BY Robert V. Kesling iii I 64~fi 4*=4ItS--44=4*O - CONTENTS

OPTICS ...... 1 Accelerator ...... 27 Light ...... 1 Restrainer ...... 28 Reflection ...... 2 Preservative ...... 28 Transmission ...... 2 Rinse bath ...... 28 Absorption ...... 3 Fixation ...... 29 Refraction ...... 3 Washing ...... 29 Diffraction ...... 4 Drying ...... 29 Lens ...... 4 Water used in making solutions ...... 30 Focal length ...... Image size PRINTING ...... Procedure Speed of the lens ...... Circle of confusion ...... Contact ...... Hyperfocal distance Projection printing ...... Types of paper Depth of. field ...... Contrasts Subject distance ...... Finishes ...... Circle of confusion ...... Weights ...... Focal length ...... Diaphragm opening ...... 9 PROCESSING ROUTINE ...... 31 Definition ...... 9 Precautions ...... 31 Lens coverage ...... 10 Darkroom layout ...... 32 Equations ...... 10 Bulk photography ...... 32 FILMS AND PAPERS ...... 19 REVISING AM3 I-KPROVING TIE NEGATIVE ... 33 Physical properties ...... 20 Factors in processing an emulsion ... 33 Emulsion and halides ...... 20 Proper exposure ...... 35 Support or base ...... 21 Negative faults ...... 35 Emulsion ...... MULTIPLE LENSES Overcoating ...... Light transmission of lens Noncurl backing ...... Stray light in lens Antihalation backing ...... Emulsion properties Illumination of field of image ...... Reversible lenses Film speed ...... Grain ...... Two-lens system ...... Color sensitivity ...... Three-lens system ...... Contrast ...... 23 THE MICROSCOPE AS A LENS SYSTEM ...... 44 Resolving power ...... 24 Brands of microscopes ...... 44 Subject brightness ...... 25 Optics of the microscope ...... 44 Negative density range ...... 25 Numerical Aperture ...... 47 Gamma ...... 25 Kinds of lenses' ...... 48 PHOTOGRAPHIC CHEMISTRY ...... 27 Lens aberrations ...... 49 Developer or reducing agent ...... 27 Combined microscope lens system ..... 50 mFERENCES ...... 51 Planning Photography Microfossils

Robert V. Kesling

PHOTOMICROGRAPHY using an optical system This is a concise list of the stages through is a very specialized kind of photography. It which each photograph must proceed. Thought- involves many problems not encountered in the ful and careful planning is required so that each photography of large specimens with an optical stage is adjusted and integrated with the others. camera or microscopic specimens with a scan- OPTICS ning electron microscope. Within the near fut- ure, however, SEM facilities are likely to be In the optical considerations -- the trans- available to only a fortunate few micropaleontol- fer of light from the subject to the film with ac- ogists, and the cost of producing large numbers ceptable clarity -- many factors are interrelat- of photographs will be prohibitive for most bud- ed. The lighting of the subject determines the gets. Commercially designed and produced op- intensity and contrast of the image, as well as tical photomicrography systems are also expen- the portrayal of the essential characteristics of sive, and most of them include features unnec- the subject. The size of the subject and its mag- essary for the particular usage intended. Hence, nification on film determine the size of the film micropaleontologists seem destined for some and the degree of enlargement in printing. The time to planning their own system, utilizing in- focal length of the lens sets the size of the cam- sofar as possible whatever equipment may be era and its support. The optical design and ac- already" -~resent in their organization- or unit. curacv of the lens and the diameter of the open- The following data and photographic "prin- ing the depth of field in the final print. ciples" are presented for use in planning and If all factors are llperfect" we have an accurate, operating a "single-lenstf (usually a coordinated exciting, and even dramatic interpretation of the subject; if all factors are planned and ex- set of lenses cemented together) camera. A few suggestions on films and printing are added ecuted with moderate care, we have an accept- for consideration, although success or failure able picture; but if any one factor is wrong, we in photomicrography centers on the design of have nothing for our efforts except the experi- the camera. ence in what not to do. - In "Photographic Fundamentals" (1961, Light. -- Let us begin the discussion with Course 2320, Extension Course Institute, Air light, since light is the essential denominator in all phases of photography. We are by necessity University, Gunther Air Force Base, Alabama), photography is described (p. 57) as involved with the nature, properties, quality, and behavior of light from the time it bounces ... a process made up of many miracles .... off the subject, makes its way through the lens, the miracle of optics -- how sharp, detailed, and penetrates the film emulsion, until we can brilliant images can be formed by lenses and "see" the final result as a series of light waves projected on a sheet of film .... the miracle radiating from the print. of film, and the latent image *hat is formed on it .... the miracles of chemistry, by which Light has dual sets of characteristics, the latent image becomes a visible one through both taking part in photography. This visible the action of a developer, and then becomes form of energy can act as a radial wave action, permanent through the action of a fixing bath. as in the waves emanating in all directions PAPERS ON PALEONTOLOGY from the subjects we see (like waves spreading specular reflection outward in a pool from the point where a pebble is dropped), or it can act as a quantum, as in the exposure of film during which an amount of light passes through the lens opening during the interval the shutter is open (like the quantity of I mirror I water passing through the spout during the time the spigot is opened). diffused reflection Sunlight contains waves or radiations of all colors in the visible band or spectrum, as well as those longer than the red end (infrared) and those shorter than the violet end (ultravio- let). That is why it is ideal illumination: what- I rough surface I ever the wavelengths to which the film is sen- sitive, they are present in sunlight. Reflections from surfaces.

The electric bulb, fluorescent unit, flash, pression that is not lighted. The film is im- or arc light used for artificial illumination has personal; it registers the colors and their in- serious gaps in the spectrum of its light. The tensities as the light energy passes through the human eye is so marvelously accommodating to lens during exposure, impartially and finally. the owner that it is really a poor judge of the colors (or even the accurate intensity) of such With rare and special exceptions, the light; the emulsion on the film is much more paleontologist seeks to portray the form of the discriminating. Before selecting an illuminator fossil and not its color or polish. Therefore, the photographer should check its registration when he photographs, he wants the specimen to on the film with the optical system he plans to give off diffused light. To bring this about, he use. Filters should be added only if they im- usually coats the specimen with a very thin prove the image on the film. layer of sublimated ammonium chloride. The particles are uniform in color (white) and make REFLECTION. When light waves strike the the surface "rough. " As long as the coating is surface of the subject, they may be reflected not thick enough to obscure any structures of or thrown back. If the surface is smooth and the fossil, and its particles are not largeenough polished (like a mirror), the reflected light is to be distinguished in the final print (conveying thrown back at the same angle as the incident a false impression of texture), the ammonium or incoming iight, and the reflection is said to chloride is successful in bringing out the con- be specuZar. But if the surface is rough or ir- figuration of the specimen. regular, the light is reflected in more than one direction and in more than one plane. Such re- TRANSMISSION. Light passing through an flected light is diffused. object is transmitted. Objects such as a lens or window pane, through which objects are As we see a microfossil under the micro- clearly visible, are transparent. Frosted glass, scope, we mentally adjust the image to empha- which scatters the transmitted light so that ob- size what we wish to "see." If a specular re- jects cannot be seen clearly, is transzucent. A flection tends to obscure some detail, we sim- medium which will not transmit light at all is ply ignore it ("filter" it out mentally) or we said to be opaque. turn the specimen so that the polished surface reflects in another direction. If the fossil is Transparency is a matter for considera- transparent, we focus our attention on the out- tion with the lens. In old lenses, the cement ermost surface. And we subtly distinguish to between the lens elements may become dis- ourselves whether a dark spot is a stain on the colored with age or lose some of its transpar- surface, the shadow of an elevation, or a de- ency; in either case, the lens is seriously im- PAPERS ON PALEONTOLOGY

Direction of refraction, paired in transmitting light faithfully, and it cannot readily be mounted above the background. should not be used in such condition. The blackness of the background must be added on the plates with "photographic black" show- ABSORPTION. When light falls on an object card color (Rang #834 Black Tempera Color and is neither reflected or transmitted, it is diluted slightly is a satisfactory and cheap absorbed. If our plates are to have a black substance). This involves carefully outlining background, absorption is a factor to be reck- each printed picture with the black solution, oned with. With large fossils, the specimen but the process seems to be unavoidable. can be mounted on a small pellet of clay or wax to elevate it above the black-card back- =FRACTION. Another important property ground; then when the specimen is coated, the of light is that it can be "betit" as it passes ammonium chloride settling on the background from one medium obliquely into a substance of can be wiped off with a moistened brush. Ex- d13erent density. The bending of light is called posed and developed photographic printing paper refraction. As a light ray passes at an angle (even with a matte surface) reflects some light. from air into glass (denser than air), it is bent Black illustration board absorbs light far better toward the perpendicular to the air-glass sur- and its rough texture tends to diffuse any re- face; and as the ray emerges from the glass in- flected rays. With microfossils, the specimen to air, it is bent away from the perpendicular. PAPERS ON PALEONTOLOGY

focal B - focal plane with nearer than infinity plane focus A \ \ parallel light rays from infinity from point ----- at infinity -I-*c

( focal length I light rays from point A - focal plane closer than infinity with infinity focus Focal 1ength . Relation of subject distance to focal plane

By refraction, a lens gathers light rays from When a ray passes through a pinhole, all distant subjects and concentrates them on the the perimeter of the hole is causing diffraction film. of the light. The only way to avoid any such Let us consider light rays radiating from scattering would be to make the pinhole in a a point source and a camera directed toward metal plate of infinite thinness. Since this is that source. If we replace the lens with a pin- impossible, the best way to reduce diffraction hole, light can enter through it into the camera from the diaphragm of a lens is to use the thin- and strike the film. If the light source is far nest metal suitable and to avoid very tiny open- away, the ray reaching the film will be scarce- ings. More will be said about this later under ly larger than. the pinhole through which it en- "Definition. tered the camera. But if the source is quite Lens. -- The lens is a device made of close to the camera, light rays will spread high quality and uniform glass so shaped that through the pinhole as a cone of illumination, the light rays from any point of the subject, and the point source of the light will be repre- irrespective of where they fall upon the lens' sented on the film by a circle. This will be surface, will be refracted to meet and form an referred to later as the circle of confusion. image of the point. The point where these rays Now consider two pinholes several milli- converge (actually, the very small area where meters apart. Rays from the distant point they come together) is called the focus, and the source will enter each hole and register as two plane on which the lens forms the sharpest im- spots on the film. Yet if we center a lens be- age is known as the focal plane. Here the film hind the two pinholes, it will be possible to re- is placed for exposure. The function of any fract the two rays so that they coincide at a lens is to project the image of a subject on the spot on the film. If the lens is formed correct- focal plane. ly, the rays from the point source to every The lens is part of a system which incor- site on the lens surface can be refracted in porates a diaphragm, or adjustable opening, such a way that they all converge on the film -- and a shutter, or valve, The diaphragm may which is said to be then at the focal ph. be made larger or smaller to control the How closely all these images coincide deter- amount or intensity of the light being admitted mines the "sharpnesstf of focus and the value to the focal plane. The shutter is opened for of the lens. If the light passes through more an interval of time to admit this light during than one lens on its way to the film, any inac- exposure, and thereafter is closed to prevent curacies in the curvature of any of the lenses additional and unwanted light from entering the will affect the image at the focal plane. Hence, camera. The longer the shutter is open, the a single lens may yield better results than a more the quantity of light which passes through series of lenses. it. The film is "illuminatedfr only when the shutter is opened, and must be kept in total DIFFRACTION. AS light strikes the edge darkness before and after the exposure until it of an opaque medium, it is scattered slightly. is developed in the darkroom. When lamplight passes by an opaque object, the shadow of the object has a "fuzzy" edge. Focal lenpth. -- The distance from the center of the lens to the focal plane when the length lens) the scene to be photographed is con- lens is focused on a point at infinity is the focal densed onto the frame of 35-mm film; in a press length. Three factors determine the focal camera (with a long focal length lens) the scene length of a lens: curvature of the lens surfaces, would be enlarged over the area of a 4 x 5-inch the kind of glass used in its manufacture, and, sheet of film. in the case of "compound" lenses, the separa- The image size can also be increased by tion between the lenses or lens elements. moving the lens closer to the subject. If it is Rays from pints closer than infinity impossible to move the camera close enough to strike the lens at increasingly greater angles the subject to register the details you desire, and come to focus farther from the lens, be- you can substitute a lens of longer focal length. hind the focal length. This change in the pos- Hence, a telephoto lens (with very long focal ition of the focal plane results from the fact length) could be used on a 35-mm camera to that the lens will refract rays by only a certain photograph details of ornamentation on a dis - amount from the angle of incidence. As the tant cathedral. In photomicrography, the ad- subject is situated closer to the camera, the vantage of a longer focal length is the increas- lens-to-film distance must be increased. And ed "working distance" between lens and sub- if the subject is very closeto the lens, the cam- ject at the same magnification -- more room era must be lengthened by some kind of long in which to manipulate the subject and adjust bellows. the lighting. Image size. -- Provided the subject re- Speed of the lens. -- The speed of a lens mains the same distance in front of the lens, is determined by two factors: the diameter of the size of the image is directly proportional to the aperture through which light passes into the the focal length of the lens. The longer the foc- camera (controlling the amount of light admit- a1 length, the larger the image size; if the focal ted in a given interval of exposure) and the foc- length is twice as long, the image will be twice al length of the lens (governing the distance the as high. In a 35-mm camera (with short focal light must travel from the lens to the film). A

image projected to focal plane

Images on focal plane are proportiona to focal length

Relation of focal length to image size. PAPERS ON PALEONTOLOGY larger opening admits more light and is "fast- down from one half stop to the next standard er"; the longer focal length decreases light in- stop reduces the light intensity by about one- tensity because the light must travel farther fourth, but closing down from one half stop to and is spread over a greater area of the film. the next half stop cuts the light intensity in half. This becomes clear as we square the half stops: The speed is indicated by f -stops, also 11.25, 20.25, 39.69, and 90.25. These squares known a's diaphragm settings or f/numbers, are close to the idealized 12, 24, 48, and 96. which are simply ratios of the focal length to the diameter of the opening. Thus a lens with In actual practice, most cameras have focal length of 96 mm and a diameter of 12 mm stops that are accurately calibrated at f/4, f/8, has a speed of 96/12 = f/8. Similarly, a lens or f/16, and the other stops are set exactly with a focal length half as long and the same where the light is decreased or increased by diameter would have a speed of 48/12 = f/4. one-quarter or one-half. Thus some of the labeled f -stops and the actual f -stop openings The above speeds refer to lenses with the may differ slightly: aperture wide open. However, lenses are equip- Labeled - 1 1.4 2 2.8 3.5 4 4.5 5.6 6.3 ped with an iris diaphragm which can be adjust- Actual 1 1.41 2 2.83'3.46 4 4.90 5.66 6.93 ed to control the light entering the camera. The - Labeled - 8 9.5 11 16 22 32 45 64 standard f -stops on the diaphragm scale are: Actual - 8 9.80 11.3 16 22.632 45.3 64 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90. Scarcely any lens covers all this range Circle of confusion. -- Any subject being of f -stops. Only special lenses have a maximum photographed is composed of an infinite number opening of f/l, since the diameter of the lens of points, each of which reflects light toward would have to equal its focal length. On the oth- the camera. The lens creates an image of each er hand, lenses used in scientific work may of these points comprising the subject upon the stop down to f/45. film in such a way that their relative brightness will convey to the eye observing the print the m---I wo important facts should be remember - essential features of the subject as it might have ed about f-stops. First, they are fractions re- been seen directly at the time of the photograph- presenting a ratio between focal length and dia- ing. In short, the purpose of a camera lens is meter of the aperture. Thus, a setting of f/ll to produce a picture that clearly looks like the represents an aperture that equals one-eleventh subject. of the focal length, and therefore is smaller than an opening of f/8. The larger the f-stop Not even the lens in the human eye can number, the smaller the opening. focus on subjects at all distances simultaneous- ly. When the camera is focused at a certain Second, as the lens is "stopped down" distance, all the points in the subject at that (made smaller) from one standard f -stop to the distance will be registered on the film with next, the light entering the camera is reduced reasonable accuracy, and when the film is de- by one -half. Obviously an opening of f/16 has veloped they will be recorded as microscopic half the diameter of an opening of f/8, and has specks, closely corresponding in size to the only one-fourth the area through which light grain of the film emulsion. may pass. This relationship becomes clear as we square the f-stop numbers in the standard point of critical focus series above; the squares are: 1, 1.96, 4, where spread is least 7.84, 16, 31.36, 64, 121, 256, 484, 1024, 2025, 4096, 8100. This series is not far from the ideal series formed by doubling: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, '-(area of acceptable focus 4096, 8192. where spread Some diaphragms are also calibrated for from single point does not exceed 0.25 mm half stops at 3.5, 4.5, 6.3, and 9.5. Closing Circle of confusion. PAPERS ON PALEONTOLOGY

if this circle of point A confusion is .25 mm focus here or less, all subjects point B focus here between A and B will B-A be sharp

LONG SUBJECT DISTANCE point A focus here,

B-A

subject has been moved closer to the lens

SHORT SUBJECT DISTANCE this circle of confusion is greater than above; therefore, majority of subjects between A and B will be fuzzy

Relation of subject distance to depth of field. Points in the subject (or other subjects) smallest the lens can produce, and the spread which are farther away or closer to the camera will depend upon just how accurately the lens will be projected onto the film as small circles can bring all the rays of light from a point to of greater diameter than those at the focalpoint converge on the focal plane. This circle is then -- the diameter increasing as the point of con- called the rneasurabZe circle of confusion and vergence gets farther away from the focal point is a fixed property of the lens. No lens is per - for which the lens is set. This small circle is fect. However, most lenses for scientific work the circle of confusion. have a measurable circle of confusion that ap- Jf the actual measurement of this circle proaches 0.07 mm. of confusion is 0. 25 mm or less in the print, it Depending upon the design and care in will be interpreted by the eye of the observer manufacture, a lens may give a smaller or as a point when viewed from a distance of about larger measurable circle of confusion for sub- 10 inches, and the photograph is considered jects at different distances from the lens. The "sharp. " If it is larger than 0.25 mm, the eye only way to discover the properties of a parti- distinguishes it as a circle instead of a point, cular lens is to try it for your subjects at the and that part of the photograph appears blurred, necessary distance to produce the desired rnag- "fuzzy, " or "out of focus. " This acceptable nification on the film. 0.25 mm limit on acceptable circle of confusion applies to the print; if the negative is to be en- Hyperfocal distance. -- This topic may larged by four times in printing, the circle of at first seem out of place is a discussion of confusion in the emulsion of the film could only photomicrography, because it is commonly be 0.062 5 mm. To place a practical value on used in planning for distant subjects and par- negatives to be enlarged, the photographer ticularly for subjects over a span of distance. should plan for a circle of confusion of about Nevertheless, the hyperfocal distance is a fac- 0.1 mm. tor that may be used in determining the depth of When a lens is focused as accurately as field for a given lens -- and depth of field is ex- possible, the circle of confusion will be the tremely critical in photomicrography. PAPERS ON PALEONTOLOGY

circle of confusion for large aperture

Relation of aperture to depth of field. When a lens is focused at infinity, the dis- events. For example, it is found that with the tmco from the cecter of the lens to the nearest fast film being used in the camera, the light is subject in acceptably sharp focus is the hypm- strong enough to allow exposures of 1/1000th focal distance (HFD). This boundary between second at f/16. The lens has a focal length of the sharp and the blurred portions of the image 100 mm, and the circle of confusion is 0.1 mm; has a number of uses. When the camera is re- the HFD is calculated as 6250mm. Hence, focused on the HFD, the area of sharpness with the camera focused at 6.25 meters, all moves toward the camera to about half the HFD subjects will be in acceptable focus from 3.13 and at the same time, subjects far from the meters to infinity. Action shots can be taken in camera (said to be at "infinity") are still in ac- quick succession without changing any settings ceptable focus. Focusing a lens on its hyperfoc- as long as the subject is over 10 feet away. al distance gives the maximum depth of field for the f -stop being used. As shown in the dis- Depth of field. -- The distance from the nearest plane in acceptable focus to the farth- cussion of formulas, the HFD is theoretically a function of the focal length of the lens, the est plane in acceptable focus for a particular f -stop, and the acceptable circle of confusion. setting of the lens and camera is called the Actually, of course, two lenses of the same depth of fieM. It is the measurable span of focal length opened to the same f-stop may distance in front of the lens within which all yield different HFD's; one of the lenses may be subjects appear reasonably sharp on the focal superior and produce sharp images of subjects plane. Since nearly all subjects have appreci- from infinity to quite close to the camera, able third dimension, or depth, the depth of whereas the other lens may be inferior and field is extremely important in photomicrogra- scarcely give acceptable sharpness for subjects phy. As discussed below, the depth of field at infinity or any other distance. can be calculated for lenses of average grade. Several factors are concerned in this The common use of HFD is in pre-plan- theoretical depth of field. ning a rapid series of action shots for sports PAPERS ON PP.LEONTOLOGY 9

SUBJECT DISTANCE.AS the subject is far - of very short focal length in a minicamera has ther away (the distance from the lens to the such depth of field that most manufacturers do subject is increased), the depth of field increas - not even provide such camera with a focusing es. The more distant the subject, the more device. These minicameras are "in focus" from nearly parallel are the light rays reaching the a few feet to infinity. However, the image pro - lens, and all these rays tend to converge in duced by such lenses is very small because of nearly the same plane. On the other hand, the the very short focal length. closer the subject (the higher the magnification of the image), the greater the angle at which Photomicrography has very different goals the light rays reach the lens, and the greater than general outdoor photography. Instead of a the cone of spread -- and therefore the less slide or print of a distant scene, we are con- cerned with deriving a sharp image of given depth of field. magnification on the film. As will be demon- CIRCLE OF CONFUSION. The circle of con- strated in the formulas below, there is theoret- fusion is directly related to the theoretical ically no difference in depths of field for sub- depth of field and to the actual depth of field. jects taken at the same magnification and at the In the theoretical depth of field (the formula by same f -stop -- regardless of the focal length of which calculations can be made), the acceptable the lens used. size of the circle of confusion on the film image is directly proportional to the (accepted) depth DIAPHRAGM OPENING, $-STOP, OR APERTURE. The easiest and most frequently used method of con- of field; hence, the more "tolerance" of a slight- trolling depth of field is by the size of the aper- ly blurred image, the greater the span which ture. All other factors kept constant, the small- can be classified as within the depth of field. er the lens aperture, the greater the depth of In the actual depth of field (the concrete field. Stopping down the diaphragm opening results produced by the particular lens), the makes all the light rays entering the camera subject in the vertical plane on which the lens is more nearly parallel, causing them to focus on focused will have all points of its surface form approximately the same focal plane. images on the film composed of the smallest circles of confusion which the particular lens is As a standard procedure in photomicro- capable of producing. Subjects in front or behind graphy, focus with the lens wide open on the this vertical plane will register on the film in middle of the desired depth of field. Then close ever-increasing circles until the image is unac- down to the smallest opening practical for the ceptably "fuzzy. " Therefore, the more nearly particular lens, and check the actual depth at perfect the lens, the smaller will be its measur- that position of focus. If the actual depth of field able circle of confusion and the greater will be includes the closest spot of the subject that you wish to have in focus, then the camera is set its depth of field. for the exposure. F~CALLENGTH. AS just mentioned, the With all other factors equal, lenses with depth of field depends to a significant degree upon the minimum size of the circle of confus- the same f~stopshould give the same depth of field at the same magnification. Yet, as we ion that the lens can produce -- its measurable shall see below under "Definition, " all other circle of confusion. This is an inherent proper - ty of the lens and cannot be changed. A lens of factors are not quite equal, and the very short average quality has a measurable circle of con- focal length lenses do not give as deep a field fusion that is a~~roximatelv1/1000 th of its foc- at very small openings because of diffraction. al length. ~he~eiore,an a;er&e lens of short Definition. -- The ability of a lens to re- focal length will give a smaller circle of con- produce fine detail is its definition, or resotv- fusion and have more depth of field than a lens ing pmer. Two factors influence definition: the of longer focal length (at the same f -stop). quality of the lens and thk diffraction of light as This is true for subjects at a fixed dist- it strikes the edge of the diaphragm. As stated ance. For general outdoor photography, a lens before, the resolving power of any lens depends PAPERS ON PALEONTOLOGY

0 . 0 0

0 .. . 0 3 0 . '1 .. 0 U . z .. . '1 . 5 Y angle of view -1Y Ici E . .. 0 . .. aJ 0 Y 0 .. 0 .. 0 0 . r( . .. '1 .. 0 I v '. . 1ens ..

Usable portion of the circle of illumination. on its measurable circle of confusion, which is short focal length lenses, they are to be avoided inherent in its manufacture. The design of any in photomicrography except for high magnifica- lens is never perfect, so the complete lens does tions which cannot be otherwise achieved. not have precise focus of subjects positioned at various distances. Especially the edges of the Lens coverage. -- Light passing through a lens, where refraction must be maximum, have lens is projected to the focal plane in a circular circle illwnination. geometric deficiencies or aberrations which shape called the of The greatly affect definition. Therefore, most lens intensity of light decreases outward from the units work better when stopped down from their center of this circle as the distance fro= the maximum opening. lens increases. Of course, the film cannot be "dished1' into a concavity to make it parallel to When the diaphragm is stopped down to a the lens surface. To a;oid serious problems very small aperture, the diffraction of light of light intensity "fading out" toward the edges, glancing off the edges of the diaphragm causes only the center of the circle of illumination is loss in definition. This problem is acute in the used as the lens coverage on the film. To be smaller openings (f-stops) of short focal length sure that the central part covers the film, the lenses. For example, a lens of 16-mm focal rule is applied that the focal length of the lens length reaches a stop of f/45 when the opening should ordinarily be about the same size as the is only 0.36mm in diameter. This is too small diagonal measurement of the film used. Thus, for g~odresolution. a camera using full-frame 35-mm film (about This, then, is the dilemma of the photo- 25 x 35mm) should have a lens of at least 43 micrographer: to keep the camera in reasonable mm focal length -- (25 x 25) + (35 x 35) = 1850 dimensions he needs a short focal length lens, = 43 x 43. This rule must at times be violated in arriving at higher magnifications, but the and for depth of field he needs a small aperture; but the small aperture in a short focal length problem of "dim edges" must be anticipated. lens is so small that diffraction spoils the resol- ution and therewith the depth of field. Equations. -- In the following sections, some of the equations dealing with optical pro- The best resolution for any lens is a com- perties of lenses and exposure are collected as promise between closing down the aperture to a handy reference for the micropaleontologist take advantage of the fewer aberrations in the and would-be photomicrographer. For each middle of the lens and opening the aperture to topic, an example or two is given and the gen- avoid diffraction on the diaphragm edges. Be - eralized principles are stated. cause of the diffraction problem with extremely PAPERS OF4 PALEOiaTOLOGY 11

...... The magnif ica tion is inverse1Y propor ti~~22to the difference between subject distance an6 KEY focal length. rZSA = film speed The minimum magnification is achieved when s = infinity; the maximum magnification is achieved b = distance from lens to film when s = f (although this would require a cam- B = distance from infinity position to film era of infinite length). The magnification is directly proportional to the c = acceptable circle of confusion difference between the lens-to-film distance d = diameter of lens opening; the f-stop and the focal length of the lens. e = relative exposure time Example 3: f = 100 mm (The camera is focused on distant subjects at 100 nun) E = practical relative exposure time For a magnification of x 2, the lens must be Ev = exposure-value extended: 2 = - B = 200 mm f = focal length of lens 100 ' HFD = hyperfocal distance, the "fixed focus" For a magnification of x 8 (an increase of 4 position which produces the greatest times) : depth of field from infinity with an B = 4 x 200 = 800 mm acceptable definition For a magnification of x 10 (an increase of 5 m = maanification on film times): B = 5 x 200 = 1000 mm (1 meter) RI = maximum distance beyond plane of accurate focus with acceptable definition With the same lens, magnification increases in direct proportion to the bellows extension from RZ = maximum distance in front of plane of the infinity psition. accurate focus with acceptable defini- tion ...... R1 + RZ = depth of field DISTANCE FROM LENS TO FILM ("CAMERA LENGTH") s = distance from lens to subject t = actual exposure time in seconds b=f+B, [Equation #3] v = f-value; the diaphragm setting b=fm+f=f (m+1) [Equation #4] V = effective f-value Example 1: f = 100 mm (Lens has focal length of 100 m) m = 2 (Magnification of x 2 desired) MAGNIFICATION b = 100 (2 + 1) = 300 mm (Lens must be 300 mm from the film to give desired magnifica- [Equation #1] tion) Example 2: f = 16 mm (Lens has focal length of b-f [Equation #2] 16 mm) ='=f m = 2 (Magnification of x 2 desired) Example 1: f = 100 mm (Lens has focal length of b = 16 (2 + 1) = 48 mm (Lens must be 48 m 100 m) from the film) s = 150 mm (Subject is 150 mm in The "camera length" (distance from lens to film) front of the lens) is directly proportional to the focal length m = 100/150-100 = 2 (The image is of the lens. x 2 on the film for this given f and s) The "camera length" is directly proportional to Example 2: With the same lens as in example 1 the magnification plus 1. than (f = loo mm), The "camera length" cannot be less the b = 150 mm (The film is 150 mm from focal length of the lens (for the magnifica- the lens) tion would then have to be less than zero). For high magnifications, short focal length m = 150 - loo- - (The image is half 100 2 lenses are necessary for practical considera- the size of the subject) tions in the length of the camera. PAPERS ON PALEONTOLOGY

Example 1: f = 100 mm = s-f (from Equation #1) m=x4 1 100 1) 125 mm (Subject must be s = (-4 + = 125 mm from the lens to give magnification of The closer the subject, the longer the lens to x 4) film distance required for focus. The higher the magnification desired, the clos- Example 3: f = 16 (Lens of 16-mm focal length) er the subject must be to the lens. b = 640 mm (Maximum lens to film dis- The lens can never focus on subjects closer tance possible with the bellows than its focal length (for if m = infinity, extended in the camera) then s = f). What is the maximum magnification? b = fm + f (from equation #4) b = f (m + 1), 640 = 16m + 16 s-b=f - m==62416 3.9 (The camera is too short to m fm 1 reach a magnification of x 4) s = frn + b = f (; m) b [Equation 181 mf - - + The maximum extension of the camera determines the upper limit of magnification for a partic- Example 2: For the same lens (f = 100 mm) and ular lens. magnification (x 4) as in example 1, b = fm iJ@ ((from equation #4) b = (100 x 4) + 100 = 500 mm 100 400 500 125 mm (Subject must be s = -4 - + = BELLOWS EXTENSION 125 mm from the lens to give x 4) B=b-f=& [Equation #5]

Example 1: f = 48 xmn (Lens focuses on distant subjects when b = 48 nun) m = b+ (from equation #2) 1 s = 96 nun (Subject 24 mm in front of 2 b&f b&f f f = + f [Equation # 91 the lens) m & B=-=48 48 48 mm (The lens must be ex- '' - 48 Example 3: f = 48 nun (Focal length of lens) tended by 48 mm to bring the b = 96 mm (Lens-to-film distance) subject at 96 mm into focus) s=- 48 48 + 48 = 144 mm (With a 48-mm lens 96 48 Example 2: With the same lens as in example 1 - in a 96-mm camera, the (f = 48 mm) and subject comes into focus at 144 m in s = 72 mm front of the lens) 48 48 B=-= x 96 mm (Lens must be extended 72 - 48 The subject distance in front of the lens is by 96 mm from the infinity proportional to the square of the focal length position to bring the sub- and inversely proportional to the difference ject into focus. between the camera length and the focal length The higher the magnification (the closer the of the 1ens. subject to the lens), the longer the bellows extension, m = + (from equation #l) f-VALUES ( "STOPS" j s The "stops" or settings of the diaphragm [Equation #6 1 are values of is The distance of bellows extension directly [Equation #lo] proportional to the magnification.

Or settings are computed the ~\\\\\\\\\\\\\\\\\\\\\\\\NB ratio of focal length to diameter of the== lens SUBJECT TO LENS DISTANCE opening . Example 1: f = 100 mmt [Equat&on #71 m d=50m P~.PER~ON PALEONTOLOGY 13

100 v = = 2 (A very high f-value; a lens tive f-value and the less light admitted to -50 set at f/2 is "wide open" and the film. many lenses are not provided The longer the focal length of the lens, the with such a large opening) 1ess the effective f-val ue. When the lens is focused at infinity, b = f Example 2: f = 48 mm and V = v. d=lm A short focal length lens will yield a higher effective f-value with the camera set for the v = 48 = 48 (A very low f-value; a lens 1 set at f/48 is "stopped way same lens-to-film distance; since the magni- down." Note that high numbers fication is higher, the effect is the same as for v are usually referred to as "low" closing the lens to a higher stop number. f-values because a lens with that setting admits a low amount of light in a given f (from equation #4) interval) v = v(m + 1) For a fixed opening (d) , the f-values vary in e = (1 + rnj2 (see equation #23) of 2 direct proportion to the focal length the e=-V lens: the shorter &e focal length, the small- v 2 er the f-value numbers for a given d. f/16 (1 + m212 for a lens with focal length of 48 mm (d = t2= 2 tl (see equation #24) 3 mm) is the same size diaphragm opening as (1 + ml) f/32 for a lens with focal length of 96 m. 2 v22 v1 t2= (T X tl s\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\K 1 v2 DIAMETER OF LENS OPENING Exposure time varies with the square of the effective f-value divided by the square of [Equation #11] the actual f-value. Example 2: In a camera, a 48-mm lens stopped Example 1: f = 100 mm down to f/16 is focussed at 96 mm from the v = 32 film; proper exposure is found to be 10 sec- onds. What is the exposure time if a 16-rmn lens stopped down to f/16 also is substituted Example 2: f = 100 mm (other factors constant) ? v = 32 100 d = - = 6.3 mm 15 v2 = 16 16 x 96 For a given lens, the diameter of the lens open- b2 = 96 mm V2 = -= 96 ing is inverse1y proportional to the f-values f2 = 16 mm 16 ("settings"). 96 96 256 t2= (- x - x = 9 x 10 = 90 seconds ...... 32 32 The same solution could also be derived from EFFECTIVE f-VALUE the magnifications: vxb [Equation #12] 1 "=-T 48 96- - 16 Example 1: v = 22 (Diaphragm set at f/22) m2=-=5 16 f=48mm b=96mm (1 + m2122tl = (6)2 10 = 90 seconds. t2= - V==22 96 44 (With this lens-to-film dis- (1 + ml) (2)2 48 tance, the camera functions as though it were set at f/44 when %\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\m compared to its operation when focused at infinity) The higher the magnification (the longer the lens-to-film distance) , the higher the effec- 14 PAPERS ON PALEONTOLOGY

DEPTH OF FIELD length has the identical depth of field as a lens of short focal length when used at the Depth of field = R1 R2, where + same f-stop to produce the same magnification CS R1 = - and [Equation #I31 of image, Any difference in depth of field md - c ' at the same magnification must be due to the CS R2 = [Equation #I41 exactitude with which the lens is ground and md- + c polished. Example 1: c = 0.1 mm (Acceptable circle of con- For practical purposes, fusion of 0.1 mm on film) f = 100 mm (Focal length of lens) Depth of field = R1 + R2 = 2cs [Equation #I61 -md s = 3280 mm (Subject 3.28 meters from lens) Example 4: Using the same set-up as in example 1, v = 4.5 (Diaphragm setting at f/4.5) 2 x 0.1 x 3280 - 656 Depth of field = --= 937 0,70 .70 Example 5: Using the same set-up as in example 2; 28.8 Depth of field = -12 = 2.40 mm

If the subject is fairly close to the lens, md is large compared to c and the depth of field can be approximated as 2cs/md. 0.1 x 3280 - 328 R2 = - = 410 mm (Subject in 0.70 + 0.1 -0.8 acceptable focus from 3280 - 410 mm to 3280 + 546 mm, or HYPERE'OCAL DISTANCE from 2870 rmn to 3826 mm) HFD = .E [Equation #17] C Depth of field = 546 + 410 = 956 mm at this s, m, and d. This formula is derived as follows: ------CS R1 = - (from equation #13) 1 md - c (from equation #7) s = f (; + 1) At R1 = infinity, md - c = 0 and md = c. d = f (from equation #11) (from equation #1) 1 cf(, + 1) For practical purposes, s - f = HFD, since f is [Equation #15] insignificant compared to the subject distance R1 = m involved, Hence, P Example 2: = 0.1 m = 2 m=L c mm HFD f=96mm v = 16 &=c=2 HFD HFD = C

RI + R2 = 2.40 mm (Depth of field) Example 1: f = 100 mm d = f/16 = 100/16 Example 3: Keeping the same acceptable circle c = 0.1 mm of confusion, magnification, and diaphragm HFD = loo loo= 6250 mm = 6.25 meters opening, but using a lens with 0.1 x 16

2 4 R2 = = 1.14 mm Hence, objects are in acceptable focus from 2.1 3.13 meters to infinity with this lens set R1 + R2 = 2.40 mm at f/16 and focused accurately at 6.25 m. Comparing examples 2 and 3, we note that The hyperfocal distance decreases (the lens pro- For a given magnification and f-value, the depth duces a greater depth of coverage) if the lens PAPERS ON PALEONTOLOGY

has a shorter focal length. However, it must be remembered (see equation #1) that a lens of shorter focal length also produces a small- er image (m) on the film. This is the prin- Or solved as ciple on which the mini-cameras are designed; = 1.20 m. if the focal length of the lens is very short, m HFD 2 x 5760 the camera has nearly universal focus. Example 3: Using the same set-up as in example If we accept a larger circle of confusion, say 3 under "Depth of Field": mrn, 0.25 the hyperfocal distance decreases. c = 0.1 mm m = 2 The image will be "fuzzier" at infinity and f=16m d=lm at the R2 position, but will cover a greater span of distance. Decreasing the aperture (d) will give closer Using equation #19 -- hyperfocal distance and increase the span in acceptable focus. Hence, with a fast film, mf,f m + f, RI = R2 = - 8(8 + 16) = 1-20 * short focal length lens, and small f-stop, HFD 160 photographs could be taken with a fixed-focus camera that would give acceptable definition A more accurate approximation of the depth of from less than a meter to infinity; the draw- field is: back is the small size of the image produced. R] =%m HFD ad [Equation #21] Once calculated, the hyperfocal distance can be R2 = A-j [Equation #22] used to find the depth of field, thus: m HFD + R~ = R~ = CS (from equation #16 md Example 4: Using the same set-up as in example HFD -- fd/c (from equation #17) 3 under "Depth of Field": sf c = 0.1 m m=2 md m x HFD f=16mm d=lmm [Equation #I81 s=24m mm Another useful formula for depth of field invol- HFD = 160 ving hyperfocal distance is derived: m=s-f (from equation #1) R1 =n2=A= rn HFD = As the hyperfocal distance decreases, the depth (HFD) of fie1d becomes grea ter . R1 = R2 = s(s - f) [Equation #I91 HFD &\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ Still another, using f, m, and HFD: RELATIVE EXPOSURE TIME s = f (from equation #7) For all magnifications, relative exposure rnf + e = (1 + rn12 [Equation #231 f(fm m + f) Example 1: For subjects at infinity (rn = 0) the R1 = R2 = [Equation # 201 HFD exposure is 1/50 second. e = (1 + o)~= 1 Example 2: Using the same set-up as in example For natural size on negative (m = 1) with the 2 under "Depth of Field": same light, c = 0.1 mm m=2 e = (1 + 1)* = 4 (The exposure is 4 times as long, and the time will be 4 x 1/50 = 2/25 sec- ond) The formula can be written for actual time as: [Equation $241 l&ni ni + n - Rl = R2 HFD Example 2: m = 2 and tl = 10 (Exposure is 10 seconds for magnification of x 2) PAPERS ON PALEONTOLOGY

PRACTICAL EXPOSURE GUIDE FOR CHANGING MAGNIFICATIONS

Changing to magnification of -- x0.5xO.75~1 x1.5 x2 x3 x4 x5 x6 x8 x10

Example 1: Correct exposure at x 1 is 3 seconds. Example 3: Correct exposure at x 10 is 1 minute. Exposure at x 8 = Exposure at x 2 = 64 x 3 = 192 seconds (1 min 12 sec) -04 x 60 = 2.4 seconds

Example 2: Correct exposure at x 4 is 50 secs. Example 4: Correct exposure at x 0.5 is 1/25 Exposure at x 0.5 = second. Exposure at x 8 = .016 x 50 = 0.8 second 256 x .04 = 10.24 seconds

For magnification x 5 with the same lens, light, the exposure time in changing magnifications and f-sto~: is increased or decreased by the ratio of the m2 = 5 square of the new magnification to the square of the old magnification. t2= + 5, x 10 = 2 x 10 = 40 seconds (1 + 2) 9 If the magnification remains constant, the time The relative exposure is proportional to the changes yith the f-value changes, as square of the magnification + 1. t2= v i 3xtl [Equation # 261 "1 Example 2: tl = 10 seconds PRACTICAL EXPOSURE TIME v; = 11 (d = f/li) v2 = 16 (d = f/16) For changing magnifications at natural size and as: t2= 256 x 10 = 21.1 (about 21 seconds) above, it is better to compute exposure 121 . E = m2J and [Equation #25] Regardless of the focal length of the lens used, the time of exposure increases or decreases by the ratio of the square of the new f-value to Example 1: ml = 2 and the square of the old f-value. t 1 = 6 seconds. The higher the f-value (the smaller the diameter m,- = 8 of the diaphragm opening), the longer the ex- 8 posure time. t2= -X 6 = 2 x 6 = 96 seconds. 2 4 For magnifications at natural size and ahve, PAPERS ON PALEONTOLOGY

PRACTICAL EXPOSURE GUIDE FOR CHANGING f-VALUES

Changing to f/ -- 2.8 3.5 4 4.5 5.6 6.3 8 9.5 11 16 22 32 45

Example 1: Correct exposure at $/4.5 is 3 secs. Example 3: Correct exposure at f/4 is 1/50 sec. Exposure at f/22 = Exposure at f/45 = 24 x 3 = 72 seconds 128 x .02 = 2.56 seconds Example 2: Correct exposure at f/5,6 is 8 secs. Example 4: Correct exposure at f/32 is 40 secs. Exposure at f/16 = Exposure at f/6.3 = 8 x 8 = 64 seconds -042 x 40 = 1,68 seconds ...... EXPOSURE-VALUE This factor relates the light admitted dur- Briefly, Ev acts as a value on an exposure meter ing an exposure adjusted for the film speed. Actually some -t exposure meters are calibrated in Ev numbers. v 2 With the f-value (v) and time (t)remaining con- to the speed of the film (ASA) stant, Ev varies with the negative logarithm of the film speed (ASA) . The higher the ASA [Equation #27] rating, the lower the Ev (other factors being constant); this is logical to reason through, Ev is an exponent of 2 in the formula. It can since with a fast film less light is needed be shown to be: to produce the same density of registration 25 x v2 on the film. 2Ev= - [Equation #28] t x ASA With the same film speed (ASA) and f-value- (v), Ev x log 2 = log 25 + 2 logv-logt- 1ogA.A the necessary EV for proper exposure is great- log 25 + 2 109 v - 109 t -log ASA er with less time (t). Ev = log 2 With the same film (ASA) and time (t), the nec- Ev measures the quantity of light necessary to essary Ev for proper exposure is greater with produce a given effect on the film being used increased f-value (v), that is, with smaller in the camera. diaphragm opening, PAPERS ON PALEONTOLOGY

TABU3 OF 2 Ev VALUES

From this table other 2 Ev values can be readily computed, since Intensity of light is porportional to 2 Ev and not to Ev. Thus, if one light bulb produces an 2 Ev~+EV2= 2Evlx 2Ev2 h'v = 5, then the concentrated output of 5 bulbs Thus, 220 = 2 lo x 2 lo = 1024 x 1024 = 1048576. is calculated from 2 Ev values as : 32 x 5 = 160 = zEv, and Ev = about 7.32. And 27'2=27.0x20'2=128x 1.149 = 147.1 ~'?~'?S'?>R'iX>RRNW\\\~\\\SS&\\\iiiSR\\i\\R\\iiiWWRRiW\\wiiii\\\\\\~\\\~

Example 1: Proper negative density is obtained more convenient to have the value of 2 Ev for on film of ASA = 25 at 1 second with aperture reference). set at f/16. What is the quantity of light Example 4: In the same set-up as in example admitted during exposure? 3, what would be the time at f/22 for a film 25 x v2 of RSA = 25 with the same lighting? 2Ev = - (From equation #28) t x ASA v22 x 25 Ev = and 2 = 25 256 = 256; from the table, Ev = 8. t2 1 x 25 MAZ x 2Ev v12 x 25 Example 2: The light source is found to have an tl = Ev = 12 at the magnification being used. What ASA x 2Ev would be the proper exposure time for film of v22 x ASA1 ASA = 64 at f/22? t2= tl q qua ti on #30] v> x RSA 2 Ev = 12 2Ev = 4112 If the lighting of the photographic set-up in v = 22 v2 = 484 example 3 remains constant and the film and t=-v2 x 25 [Equation #291 f-value are changed, then ASA x 2 Ev 484 loox 10 = 301 seconds = about t2 = 64 x 25 5 minutes. Example 3: Proper exposure is found to be 10 sec- Other factors being equal, the exposure time is onds at f/8 on film of ASA = 100. What is the inversely proportional to the speed of the rating of the light source in this set-up (to =-.,- L be used for future reference)? llill . Example 5: In the set-up of example 4, what 2 Ev = 25 64 ---= 1600 1.6; from the table, 100 x 10 1000 would be the proper diaphragm setting to re- Ev = about 0.6. (For most usages, it is duce the exposure to 1 minute? TABLE OF Ev VALUES

The simplest solution is to refer to the "Practical Exposure Guide for Chanqing f- FILMS AND PAPERS . - values." In this chart, the f-value h&ing Of all photographic phenomena, the latent the equivalent of 1/5 or 0.2 is slightly be- low f/ll (read from f/22) . image is one of the most fascinating - - an invis - ible ~ictureretained in the emulsion from the moment of exposure until the beginning of dev- elopment. This amazing potential of the film or printing paper cannot be seen in any way, yet there it is, awaiting chemical reactions to make its image "real." How is this latent image re- PAPERS ON PALEONTOLOGY

TABLE OF THEORETICAL DEPTHS OF FIELD (in mm)

FOR CIRCLE OF CONFUSION = 0.1 mm x 0.5 x 0.75 x 1 x 2 x 3 x 4 x 6 x8 XI0 X20

0 f/8 9.60 3.73 3.20 1.20 .71 .50 .31 .23 .18 .08 C

FOR CIRCLE OF CONFUSION = 0.25 mm

Computed by formula for lenses of equal and ayerage accuracy: 24 1) 2cv (5 1) 2cs - + m + Depth of field = - -ma - m/v m ~~\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ corded? We can only describe it in general, are exposed in proportion to the intensity of the because even the experts in physical chemistry light striking the film. Bright light (reflected seem to be lacking a clear explanation. from a white area of the subject) will expose a thick layer of halides by penetrating the trans- properties. -- and papers lucent emuision; dim light (from dark or gray contain three essentials: emulsion, halides, and areas of the subject) will expose a very thin lay- support. er; and the light striking a black area of the EMULSION AND HALIDES. Film and paper are subject may be almost entirely absorbed, so photographically sensitive to light because they that the faint light reflected from it may not contain, suspended in the gelatin coating, mill- affect the halides at a-11, ions of tiny crystals of siZver halides. These The developer changes these exposed hal- halides change after exposure to light and there- ides into black metallic silver to form the vis- upon become of being changed to ible image. The brightest parts of the subject lic silver (which is visible) whenever immersed (which reflected light with the strongest inten- in a chemical solution known as the developer sity) will be represented by the darkest part of or reducing agent, This physical change of the the developed image, and, conversely, darkest crystals upon exposure to light is the whole bas- or shadow parts of the subject (which reflected is of photography. Yet it remains a mysterious light of low intensity) will be represented by reaction. the lightest ("thinnest") areas of the image. The silver halide crystals in the emulsion This reversal of dark-for-light and light-for- dark makes the film a negative. When a nega- tive is printed on photographic paper, the tones are agdin reversed in a repetition of the pro - cess, and the tones correspond to those in the ,overcoa ting original subject; such a print is called a posi- tive. film base 4 1 SUPPORT OR BASE. Without a sheet for sup- ()+backing port, the gelatinous emulsion would collapse, fold, and roll into a useless mass during the Section through a sheet of film. developing process. The material used as the F, but above that temperature the films must be support or base for the photographic emulsion put into a hardening bath. must have two essential qualities: (1) it must be OVERCOATING.TO prevent scratches and photographically inert (incapable of causing any harmful action to the latent image or the devel- abrasions, most film emulsions have a protect- oping image), and (2) it must remain unaffected ive overcoating of clear hardened gelatin. Paper emulsions are often protected similarly because by the chemical solutions used in processing. of the thinness of their emulsion layer. The base for films is usually some form of cellulose derivative which is transparent, NONCURL BACKING. Because the emulsion swells as it is wetted and shrinks as it is dried, tough, lightweight, and flexible, such as cellu- while the film base remains unaffected, the lose acetate. For photographic prints, paper is the ideal support, for it is durable, fairly flexi- film tends to curl one way and then the other and may permanently warp in the process. Mod- ble, and opaque (as well as economical). Films ern films alleviate this problem by having a gel- are made in different grain-size emulsions and atin backing (on the side of the film base oppos- numerous speeds. Printing paper is made in several thicknesses (weights), different textures, ite to the emulsion) that swells and shrinks at and a standard series of contrasts. the same rate as the emulsion layer and hence counteracts the strains of expansion and con- EMULSION. Photographic emulsions are of traction. two general kinds according to their sensitivity to light. Negative emulsions are very sensitive ANTIHALATION BACKING.A dye added to the and used in the camera, and positive emulsions noncurl backing absorbs any intense light that are much less sensitive and used for printing penetrates the emulsion. If not absorbed, such the negative . light rays would reflect from the back of the film, scatter, and produce a halo effect around In most emulsions for negatives, a small the bright spots of the image. The color of the percentage of potassium iodide (KI) is added to antihalation dye is either a neutral gray or the the silver bromide (AgBr) to create crystals of color to which the film is sensitive. silver bromoiodide (Ag,BrI), which are much more sensitive to light than the pure silver bro- Because the dye is highly sollible in water, mide. Emulsions for printing papers contain it is completely removed during early stages of crystals of either silver chloride (AgC1) or sil- development and never poses a problem in print- ver bromide (AgBr) or a combination of the two; ing. for enlarging, the faster "bromide" papers are Emulsion properties. -- Certain properties used. or characteristics are built into a film during manufacture. These are so varied that every Gelatin is the ideal substance for keeping film company makes numerous different types these silver halides in suspension because it in every size of film. The properties include swells when wet, allowing penetration of the pro- film speed, inherent contrast, color sensitivity, cessing solutions, and contracts when dried, latitude, graniness, and resolving power. For keeping the grains in their original positions. a particular photographic task, one or two films Gelatin requires no special treatment below 85O will offer a much better combination of the de- 22 PAPERS ON PALEONTOLOGY sired characteristics than any of the others. the grains by causing them to "clump" together. From the following factors, you can make a log- High-speed films, therefore, have inherent ical choice. graininess. When such grains are much enlarg- ed, the print shows visible (discernible) "grain" FILM SPEED. NO other property is as much that destroys the sharpness of the image. For emphasized in advertising as that of speed. Even enlargements, the slower - speed films are to be in photomic'rography, where exposures are com- preferred because of their small grain. monly measured in seconds instead of fractions of a second, being able to shorten the time is The main factor in graininess of the nega- something of an advantage. This advantage is not tive is the inherent grain size traced back to its so overwhelming, however, that it should out- manufacture, but the development of the nega- weigh other factors. A few extra seconds are tive can also increase the grain size. The so- not so important that we should sacrifice the best called "fine-grain" developers control the grain contrast or grain size. After all, the specimen size to a certain extent by preventing excessive is motionless, and fast shutter speeds and film clumping together of silver grains during pro- are hardly required to "freeze" the action. cessing. Film speed expresses the sensitivity of COLOR SENSITIVITY. Because silver brom- the emulsion to light. The faster the film, the ide is sensitive to blue, violet, and ultraviolet, less amount of light needed to produce a satis- all photographic emulsions are inherently sen- factory negative. Speed starts with the way the sitive to these wave lengths. Sensitivity to other emulsion is "cooked" or "ripened" before it is colors is made possible by adding various sen- coated onto the backing of the film. The longer sitizing dyes to the emulsion during manufacture the emulsion is processed before being spread to achieve the balance desired. onto the film, the larger the grains of silver The human eye and the film differ. The bromide, the greater their sensitivity, and the human eye sees both brightness and color, but faster the film speed. the film "sees" only degrees of brightness (in Most films are judged by standards set the black-and-white films used in photomicro- by the American Standards Association -- the graphy for reproduction in publication) in the ASA rating. This is based on the minimum ex- range of wavelengths to which it is sensitive. posure required for a good negative, and can Actually, with selected dyes the film can be be used for direct comparison of films made by made more sensitive than the human eye, dis- different cimpanies. The numbers in the ASA cerning shorter wave lengths than violet and/or rating are called exposure indices or fih longer wave lengths than red. According to speed, and are used in connection with exposure color sensitivity, films can be divided into meters or exposure computations. A film with three general classes: ASA of 100 is twice as fast as one with ASA of Orthonon fcoZorbZind) fib. Because of 50 and would require half as much exposure its limited sensitivity (blue, violet, ultraviolet), time to produce a negative of the same density. this film is not now used as much as others. It Many films have two ASA ratings, one is slow in speed, fine-grained, and very high in for daylight and one for tungsten (artificial) contrast. It was used to a great extent for copy- light. Daylight contains much more blue (to ing black-and-white drawings and charts. which nearly all films are extremely sensitive) Orthochromatic film. Hailed as the "true than does tungsten light. Therefore, the tungs- color" sensitive film, this had a greater range ten speed rating is much lower than the daylight because the dye made it also sensitive to green. rating, because the film will need an apprecia- The red colors did not register, however, and bly longer exposure with the blue -less artificial any reddish areas were little represented in the light. negative and came out black in the print. The GRAIN: Grain size is closely related to advantage of this film is that it can be process- film speed. The "ripening" process which in- ed under a dark red safelight, and for that rea- creases sensitivity also increases the size of son it is popular with amateurs. The film has PAPERS ON PALEONTOLOGY 23 fine grain, medium contrast, and medium speed worry about over- or underexposure. The only and hence serves for a variety of situations. variable which can affect the exposure in his Panchromatic fih. With additional dyes, standard set-up will be the thinness of the white film was manufactured to be sensitive to red, coating of sublimated ammonium chloride, which in addition to the other colors. Because its can give dimmer or brighter reflection with the coverage corresponds roughly to the spectrum same lighting. visible to the human eye, this film yields the CONTRAST. The difference between the high widest color response and most natural-looking and low densities of a negative is called con- interpretation of colors in tones of the black- trast. The difference in brightness in regis- and-white scale. tration of the highlights and the shadows is the Films having fairly even balance of sen- contrast of the film. sitivity to red, green, and blue are called Normal contrast would have a range of type B panchromatic. Those with higher red densities, including highlights, halftones, and sensitivity are called type C, and are used shadows. High-contrast film cuts out the half - with artificial lighting, since tungsten is strong tone range, so that light intensities above a cer- in the red color. tain value are recorded as bright highlights and all other (lower) intensities are recorded as LATITUDE. All subjects are composed of deep shadows. Such film is normally used for points or areas of differing brightness. The brightest areas are termed the highlights, the copy work of black-and-white drawings, letter'- darkest areas the shadows, and all areas inter - ing, and maps. Medium- and low-contrast films mediate are called halftones. The ability of a record a span of halftones, and are used for film to record the range of brightness values is most subjects, including microfossils. the fi7m ZakiMe. A film capable of producing a The selection of film for photomicrography long range of these values, from darkest shadow depends to a great extent on the lighting used. to brightest highlight, has a wide latitude. But If the subject is illuminated with "soft" lighting a film that can produce only a short range of (with all sides of the microfossil illuminated and brightness values has small latitude, with little with only slightly greater intensity on the high- within its halftone range. lighted side), the film should have appreciably The average panchromatic film of medium higher contrast to give comparable results to contrast has a latitude of I to 130; that is, the those obtained with low-contrast film and with highlight can be 130 times as bright as the shad- "harsh" lighting (spotlight for the highlighted ow area and the film will still record both and side and weak lights to fill in the shadow side of the specimen). all gradations of tone between. The inherent contrast of the emulsion is Latitude in black-and-white photography controlled by its manufacture, but this built-in allows a degree of underexposure or overexpos - contrast can be adjusted in the development of ure and still gives satisfactory prints. This de- the film and in the printing. More will be said gree of deviation from the correct exposure about this in the discussion of "Time-gamma. " which will still result in a "good" negative is Most films can be developed in several solutions called acceptable Zatitude . and some cause less contrast than others in the Inas much as the photomicrographer will developed image. With a standard developer, probably work out a more or less "standard" the longer the development time, the greater the set-up, with the same lighting, lens, magnifica- resulting contrast in the film. Such "playing tion, and film for numerous photographs, lati- around" with different developers and/or differ - tude will not be as important a factor for him as ent development time, however, can only change it would be for the photographer taking each shot the contrast by a limited degree from its inher - with different lighting and magnification. Once ent properties. his results show good depth and clarity, he can In printing, the selection of high-contrast keep on shooting with no particular need to ("hard") paper can strengthen the contrast in the 24 PAPERS ON PALEONTOLOGY

low -contrast negative. Conversely, a low -contr - of the central intensity of each. This occurs if ast ("soft") paper can tone down the contrast in the two lines of light are separated by at least a a harsh negative. A photographer should be distance of 1.12 wave lengths x focal length/ aware of the limits of such choices. Some nega- thef-stopof the aperture. In the table below of tives are too contrasty to print on any commer- the resolving powers of the perfect lens, oddly, cial paper, and some are too weak to be brought the larger apertures give theoretical larger out on available papers. Even "burning in" the numbers of lines per mm than do smaller aper- highlights by selective overexposure in printing tures. However, the actual lenses produced may not be sufficient to show details if the nega- have their greatest aberrations near the edges, tive is extremely dense on these spots; and even and their resolving powers are far below those "dodging" the shadow areas by selective under- of the perfect lens. Nevertheless, it could be exposure can never show details that are not useful to remember that with some lenses, the registered to some degree in the negative. It is resolving power is better at intermediate f-stop far better to achieve a balance of lighting of the openings than at very small openings. specimen and contrast in the film such that any burning or dodging is rarely, if ever, necessary in printing.

RESOLVING POWER.The ability of the emul- sion to reproduce fine detail is termed its resol- Associated film properties. -- The prop- ving power. This is commonly measured as the erties or characteristics of film are not indep- number of lines per millimeter which can be re- endent of one another. Films tend to fall into corded clearly discernible in the film. High- or between two extremes: speed films fall in the range of 50 lines or less, and certain slow films will record as many as 250 lines. Persons with normal vision resolve no more than 6 lines per millimeter, so that Speed Fast Slow only where great magnification is necessary in printing the negative does resolving power as- Grain Coarse Fine sume a significant role. Contrast Low High Resolving power is closely related to the Resolv- grain size of the film; the finer the grain, the ing power Low High greater the resolving power. Very few emul- sions are made that deviate much from the 40 Latitude Wide Low to 55 line span, and all are below the resolving ~- -- - - power of the average lens. Some very specialized films combine fast speed and wide latitude with a significantly finer grain In considering the resolving power of the than that found in the usual commercial films of lens (apart from the registration on film), cal- this kind; all require special development. culations can be made for "perfect" lenses. Be- cause of the finite wave length of light (averaging In optical photomicrography, the desirable about .0005 mm in the visible spectrum), the im- fine -grained, high-contrast, high resolving pow - age of a narrow line of light formed by a perfect er films usually are accompanied by slow speed lens consists of a broad line of light bordered on and low latitude. The latter characteristics do the sides by a series of progressively fainter not affect the operation of a "standardw set-up parallel bands of light and darkness. If two very much; the exposures are normally in seconds, close narrow parallel bands of light are trans - and doubling or even tripling the exposure time mitted by the lens, their images will consist of does not slow up the procedure unduly; and, the summation of light intensities from both. once correct exposure is established, wide lat- Their identity as two lines can be detected only itude in the emulsion is unnecessary. when the intensity between them is less than .0.8 PAPERS ON PALEONTOLOGY

Subject brightness range. -- Two factors T = transmission of the negative affect the relative and actual densities of vari- 0 = opacity of the negative ous points in the negative: the subject bright- D = density- of the- negative ness range and the densities imposed on the film by the development. 10 1 If the subject has the brightest spot that I4 = log 0 = log - = log - = - log T is to be recorded with detail only 4 times as It T bright (as could be measured by accurate light- Thus, density is equivalent to the negative log- meter) as the darkest shadow spot that is to be arithm of the transmission of the negative. It recorded with detail, the subject brightness can be measured for the thinnest and densest range (SBR) is 4, which is very low in contrast parts of the negative. A simple way to measure (said to be "flat"). For example, a meter read- the values of I, and I, would be to take a spot ing of the shadow area is 8 and the reading of reading with a light meter of the light coming the highlight is 32, the ratio is 32/8 = 4/1 and through an enlarger without the negative in the SBR is 4. If the brightest spot is 250 times place, and another reading of a projected spot as bright as the darkest spot, the subject is of the image as projected through the negative. very contrasty. Divide the first figure obtained by the second to get the opacity of that spot on the negative; From the results obtained in the negative the density is the logarithm of the opacity. during experimentation, an estimate can be made of the SBR and used in planning improve - Gamma. -- The development of the nega- ments to the lighting and exposure. tive permits adjustment of the contrast in the Negative density range. -- This topic can subject as registered in the latent image. A be discussed here perhaps better than after the numerical measure of the contrast provided by gma. section on "Photographic Chemistry, l1 because the development process is called Since the evaluation of the negative may lead to alter- time, temperature, and agitation, as well as ations in the lighting and exposure time. the composition of the developer and the type of film, affect the final contrast in the developed The negative density range (NDR) com- negative, all these factors are embodied in the pares how much light gets through the negative concept of gamma. at the thin spots (image of the shadow spots in the subject) as compared with how much light The aim of development control is to ach- gets through the negative at the dense spots ieve a negative that is (1) capable of showing (image of the highlights in the subject). The clearly as many details of the subject as are photographic computations involving density are deemed necessary to portray its essential char- made to conform with the American Standard acter, and (2) capable of being printed on com- for Diffuse Transmission Density. The explana- mercially available printing paper to show such tion of NDR is not as formidable as it appears. details to advantage. Simply, Gamma expresses a relationship between NDR = dens it^^^^,- Density (min) subject brightness range (SBR) and the negative In other words, NDR is the difference between density range (NDR): NDR the strong transmission through the thin spots Gamma = compared to the weak transmission through the log^ dense spots in the developed negative. As shown in the Table of Negative Density Using the following symbols: Ranges, a negative density range of .90 could be achieved for a subject brightness range of 4 if I = light intensity (technically, "luminous fluxf1) properly exposed and if the development process incident on the negative could be "pushed" to an extreme gamma of 1.5. I, = light intensity transmitted through the Most developers reach only as high as gamma negative of 1.1 or 1.2. 26 PAPERS ON PALEONTOLOGY

TABLE OF NEGATIVE DENSITY RANGES

Subject Brightness Range (SBR) 4 6 8 10 12 14 16 18 20 24 28 32 36 40 44

Subject Brightness Range (SBR) 48 52 58 64 70 80 90 100 120 140 160 180 200 220 240

Nevertheless, using a developer which This table is useful in adjusting develop- can attain a range of gamma from 0.5 to 1.1, ment to give the most readily printed negatives it can be seen in the table that a negative den- from the lighting-lens-aperture-magnification- sity range of .90 can be reached for a SBR of exposure set-up. Or the SBR can be adjusted 64 by developing to gamma = 0.5; or for a SBR by changing lighting so that all development can of 32 by developing to gamma = 0.6; or for a proceed at a selected gamma. SBR of 20 by developing to gamma = 0.7; or even How gamma is reached in development for a SBR of about 7 by developing to gamma = is discussed under Photographic Chemistry. 1.1 (very high). Without special papers and developing solutions, a negative density range below.50 or above 1.40 is difficult to print satis- factorily, as indicated by the dotted areas on the table. PAPERS ON PALEONTOLOGY 27

PHOTOGRAPHIC CHEMISTRY It makes a harsh contrast in the negative by building up extreme density in the exposed areas The procedure for making the image vis- of the negative. It is used alone for high-contr- ible is the same for film or paper -- passing ast reproductions of drawings, but for general the material through successively: developer, use it is mixed with met01 to subdue its harsh- fixing bath, and wash. The only difference is ness and bring out the halftones. in the chemicals used, particularly in the dev- eloper. Hydroquinone is sensitive to temperature; below 50°F it becomes inert and above 80°F it Developer or reducing agent. -- Although tends to fog. It does keep well in solution and many chemicals are capable of reducing silver oxidizes slowly in air. halides to metallic silver, only a few are sel- ective enough to reduce only the exposed hal- METOL-HYDRCQUINONE . Usually referred to ides. All others are useless, since they reduce as M-Q developer, it is versatile, having the all halides in the emulsion and turn it solid desirable qualities of both met01 (bringing out black. detail) and hydroquinone (building up density and contrast). It keeps well in solution and works Pioneer photographers (before 1851) knew faster than either of its ingredients used separ- only ferrous oxalate - - FeC20,. 2 H20. Devel- ately. opment was a messy business, because the sol- ution stained hands and (without special care) AMIDOL. This developing agent, composed the emulsion. Around 1851 it was replaced by of C6H8N20 - 2 HCI, acts very quickly. It is pyrogallol, and later, around 1880, by hydro- one of a very few developers that can be used quinone. Met01 was discovered in 1891, and without an accelerator or restrainer, but it ox- modern developers evolved thereafter . idizes and deteriorates quickly in solution. Am- idol and a preservative make a solution that PYROGALLOL. This phenolic compound -- yields satisfactory results for many negatives. C6H,(OH), -- was not entirely satisfactory as a replacement for ferrous oxalate. It quickly PARAPHEWLENE-DIAMINE . This developer - - became oxidized in solution and it also stained approximately CH3CONHC6H4NH2 (N-acetyl-o- everything with which it came in contact: hands, phenylenediamine, or o -amino -acetanilide) - - negatives, and equipment. By the amount of is often used with a preservative only (sodium alkali added in its preparation, the degree of sulf ite) to give negatives of extremely fine grain contrast produced by pyro could be controlled. where enlargement is planned. Because it is The stains left on negatives were not regarded slow acting, it is often combined with IIGlycin, " by the old-time photographers as a detriment, which also produces fine grain but does not so for it had a softening effect on the resultant readily oxidize. prints . Accelerator. -- A developing agent alone METOL. Also known in the photographic in the water solution does little or no developing trade as "Elon, " "Pictol, " or "Rhodol, " met01 because it is neutral to acid. For the reactions -- Cl4HI8N2O2 -- is an energetic dev- to proceed and for the reducing agent to work, eloper that brings out the image quickly but the solution must be alkaline. To bring this to builds .density very slowly (said to be "soft- be, an acceZerator is added. It accomplishes working1'). It has the advantages of being fairly two purposes: it energizes the developing agent independent of temperature and large quantities to perform its task, and it also softens the gela- of restrainer. Because it is difficult to obtain tin emulsion, permitting faster penetration of contrast with met01 alone, it is usually combin- the developing solution. The amount of alkali ed with other developers, such as hydroquinone. added determines the "energy" of the develop- ment. Too little alkali slows the development; HYDROQUI~~. This developer, which has too much results in very high contrast and even a composition of C6H4(OH)*,is low-energy and in chemical fog when the reducing agent is push- produces great density in the highlights while ed to the point where it starts developing unex- retaining transparency in the unexposed areas. PAPERS ON PALEONTOLOGY posed halides. Concentrated alkalis may swell er in the fixing bath to be removed. the gelatin so much that it blisters and "frills" Preservative. - - Organic developing agents at the edges. High-energy developers, such as in an alkaline solution have a strong affinity for metol, require less alkali than the slow low- oxygen; when they are exposed to air they quick- energy developers, Such as hydroquinone. ly oxidize and lose their photographic capability. Accelerators are divided into three gen- To prevent this, a chemical is added -- sodium eral types according to their strength and effect sulfite (Na2S03)-- with a greater affinity for in development: strong, moderate, and mild. oxygen than the reducing agent. Whenever the solution is exposed to air, the sulfite combines The strongest accelerators are sodium with it before the developing agents can react. hydroxide (NaOH) and potassium hydroxide The preservative not only prolongs the useful (KOH). These caustic alkalis are used with life of the developer, but it also prevents the some developers for high contrast. Their act- for mation of unwanted oxidation products, which ion is so strong that they are excluded from the can cause stains on the negative. other developers. They increase grain size in the softened emulsion by causing the silver The way a solution is to be used deter- grains to clump together. mines the amount of preservative needed. If the solution is to be mixed, used immediately, and Sodium carbonate (Na2C03)is a medium- then discarded, very little preservative is need- strength accelerator that is commonly used, as ed. Solutions used in trays require more sulfite in M-Q developers. Potassium carbonate (K2 than those used in tanks, because the exposure C03) could be used as a substitute, but is less to air speeds oxidation. Developers kept at high stable in solution and more expensive than the temperatures or much diluted also need more sodium carbonate. preservative than usual. The mildest alkali is sodium borate or Sodium metabisulfite (NaHSO,) may also borax (Na2.B407 10 H20), often used in the be used as a preservative. Because pyro is fine-grain developers. It is sometimes called more readily oxidized in alkaline solution than a "buffer" alkali solution, because it slowly and any other developer, sodium metabisulf ite is continuously releases its alkali content, keeping often used in pyro developers; it is slightly acid alkalinity nearly constant. Sodium metaborate and preserves the solution longer. (NaB02 . 2 H20)is slightly stronger, but acts in the same manner. Rinse bath. -- To remove surplus develop- er from its surface, the negative or print is The accelerator influences the graininess placed in a rinse bath. Three kinds of rinse produced by development. The more active the baths may be used: water, acid, and hardening. accelerator, the more the silver particles tend to clump together, and the larger the resulting Water simply rinses off the developing grain of the negative. solution so that it does not contaminate and counteract the fixing bath. Restrainer . - - Strangely, developers need a restrainer in addition to an accelerator, An acid rinse bath, commonly referred to because without a restrainer the reaction pro- as the "short stop, " is a weak solution of acet- ceeds too rapidly and some of the unexposed ic acid (CH,COOH). It stops all development by halides will also be converted into metallic sil- neutralizing the action of the developer, and ver. This causes fogging of the negative. The thereby prolongs the life of the fixing bath. addition of a restrainer to the solution prolongs A hardening solution for the rinse bath, the development time and reduces fogging to a composed of potassium alum (K2S04.Al2(SO4), . minimum. 24 H20) is used in tropical laboratories where Potassium bromide (KBr) is most common temperature cannot be controlled. It hardens as a restrainer, but sodium bromide (NaBr) can the emulsion. In most laboratories it is unnec- be substituted. Potassium iodide (KI) could be essary and only the acid "short stop" is added used, but it is not popular because it takes long- to the rinse bath. PAPERS ON PALEoNroLoGY 29

Fixation. -- Developing the film or paper POTASSIUM ALUM. This chemical, the same does not complete the developing process. Af- that was added to the hardening rinse bath for ter the film is developed it still carries the hal- tropical darkrooms (K2S04- A12(S04)324 H20) ides where no light struck it, as well as the is a regular addition to the fixing solution, in converted metallic silver where light originally which it serves the same general purpose. It struck it. Before exposure to light, it is neces- toughens and hardens the emulsion, which be - sary to place the film in a fiSccing bath. This sol- comes susceptible to scratches as it softens ution makes the unexposed and unaffected hal- and swells in the developing process. ides soluble in water, dissolves them from the BORIC ACID. The potassium alum added to gelatin, and leaves the image of metallic silver harden the emulsion causes precipitation of al- embedded in the gelatin support. uminum sulfite (A12(SO3) ,), a milky sludge, If the film is not completely "fixed, " at when the solution becomes neutralized. Boric least some of the unexposed halides remaining acid slows the precipitation and extends the use- in the emulsion will sooner or later upon expos- ful life of the fixer. ure to air change to metallic silver, spoiling or The f ixing solution, therefore, starts out completely ruining and obliterating the image. with hypo -- the basic chemical to do the job -- The fixing solution contains a halide sol- and other chemicals are added to keep the hypo vent, acetic acid, sodium sulfite, potassium active; they have disadvantages that must be alum, and boric acid. Each serves a purpose. overcome by still other chemicals. The com- bination that fulfills the function with fewest SILVER HALIDE SOLVENT. The most common drawbacks is the solution including the chemi- chemical used to dissolve the unexposed halides cals listed above. is sodium thiosulfate (Na2S203. 5 H20), better known as hypo. It was used as a fixing agent as TIME OF FIXATION. Films should be left in long ago as 1820, and was mistakenly thought the fixer for twice the length of time it takes to to be a different chemical, sodium hyposulfite clear them. Prints should remain in fresh hypo (Na2S204- 2H20) -- hence the nickname "hypo." for at least 10 minutes. When fixation appears It converts the halides into soluble compounds. to take too long, the hypo is exhausted and un- It would perform its task alone, but other ingre- able to perform; it should be promptly discard- dients are added to make the solution last longer ed and new solution supplied in its place. and act more efficiently . Some agitztion is needed for films and ACETIC ACID. The acetic acid in the "short prints to bring other hypo in contact with the stop" may not be sufficient to neutralize the al- surface. Films or prints stacked on one anoth- kalinity of the developer. If the developer is er in the fixing solution cannot be acted upon; still alkaline when it contacts the plain hypo, it problems of incomplete fixation may not appear begins to oxidize and causes stains. Therefore, for.some time afterward (when oxidation begins acetic acid is added to the fixing bath as well as to produce stains) and new prints or films will the rinse bath. have to be exposed and processed.

SODIUM SULFITE. This chemical, which is Washing. -- If hypo is not completely el- added to the developing solution as a preserva- iminated from the negative or print, it will ev- tive, is also added to the fixing solution. The entually decompose and attack the image. Wash- addition of acetic acid (to neutralize the develop- ing with generous agitation for at least 20 min- er) causes the hypo to disintegrate into free sul- utes will wash the last traces of the fixer from fur and sulfuric acid. When sodium sulfite is the negative; heavy -weight papers may require present in the solution, it combines with the over an hour to become hypo-free. sulfur to form new hypo; in counteracting the ad- Drying. -- Drying of negatives should verse action of the acetic acid, it prolongs and proceedwly as they are suspended in a dust- preserves the fixing solution. free room; Otherwise, dust particles may be 30 PAPERS ON PALEONTOLOGY entrapped in the moistened and soft gelatin. negative onto the printing paper will eliminate Papers are less delicate and can be dried much any problems of matching the light source, con- faster by a heated dryer to give the proper fin- denser, and lens in the enlarger to give even ish (matte or glossy) to the emulsion side. illumination of the field. Water used in making solutions. -- If pos- One of the advantages accrued from en- sible, distilled water should be used in mixing largement of 35-mm negatives is the control of all developing and fixing solutions. The pres- magnification of the specimen on the print. If, ence of foreign matter, especially coagulants, as in most studies, many or all of the photos may spot negatives as it becomes entrapped in are to be made at a certain magnification, the the gelatin. Iron in the water is particularly camera can be set once to make all negatives troublesome, for it oxidizes and uses up the to the same enlargement and the enlarger can sodium sulfite in the developer. be set once to bring all negatives to the same projected enlargement. If water is clouded with slime and sus- pended particles, it may be cleared by adding It is helpful to the reader of the final pub- 15 grains of potassium alum to each gallon of lication to see the pictures of microfossils at water; the cleared water can be decanted and definite magnifications -- all on a plate at x 25, the settled impurities thrown away. The pres- or all at x 40 -- with exceptions only to show ence of the alum does not affect either develop- "close-upt1 details. Hence, the magnification in er or fixer inasmuch as it is a necessary ingre- the final printing of the.paper must be consider- dient in both. ed before any print is made. If the plates are to be reproduced at natural size, then the mag- PRINTING nification in the print will be the magnification as it will appear in the published plate; but if The basis on which the whole photograph- the plates are to be reduced to 60% of their ac- ic performance is judged is the final print. This tual size in publication and it is desired to have is a fair basis, for a poor exposure can never the photographs published at x 40, then the plate lead to a good negative, a poor negative can must be made 100/60 of the anticipated journal never lead to a good print, and even a properly size and the magnification of the prints must be exposed print can be spoiled by poor technique. 100/60 x 40 = x 66.7. For judging clarity and So, any inefficiency or error in the entire pro- cess will show up in the print. contrast, it is a good policy to make prints and plates at publication size. Procedure. -- Printing is done by one of two general procedures: contact and projection. Another advantage of projection printing over contact printing is the opportunity to cor- CONTACT. The photomicrographer is not rect what may be termed llunavoidable deficien- apt to make negatives the same size as the final cies" in the negative. Suppose the specimen has print. For one reason, the added depth of field fine ornamentation of low relief, which can only obtained at lower magnification on film is criti- be discerned when it is lighted by a strong, low- cal to successful definition in the print. Furth- level highlight. Such high-contrast lighting will ermore, if a number of photographs are to be throw the ornamentation into proper rrrelieflrbut taken, the 35-mm film has a distinct advantage may obscure details in some highlighted areas over larger film -- devloping can be delayed of the specimen and will certainly give insuffic- until all exposures are made (once the set-up ient illumination to the shadow areas. Because is perfected), saving a great deal of time that of the necessity for special lighting to show would otherwise be consumed in changing film certain details, a "poort1negative is produced; after each exposure. We need not consider con- because the emulsion responds equally over all tact printing further; if it is needed, most read- its surface, and cannot be made selective, the ers will be fully familiar with it anyhow. highlighted areas of the specimen will be shown greatly overexposed and overly dense in the PROJECTION PRINTING . Using a standard model of enlarger to project the image of the negative; on the other hand, the shadow areas of PAPERS ON PALEONTOLOGY the specimen will be badly underexposed and otherwise be too subdued to discern. thin in the negative. To a limited degree, this harshness can be alleviated in projection print- Types of paper. -- Printing papers are ing by "dodging" and "burning in. " manufactured in several inherent contrasts, several finishes, and several weights. To bring out any registration of detail in CONTRASTS. Paper is made in numerous the thin shadow areas of the negative, the light inherent contrasts, of which numbers 1 through passing from the film image to the printing pap- 5 are widely available. Number 1 is a "soft" er can be blocked out for part of the exposure paper, reserved for printing very contrasty time by intercepting the rays with some solid negatives; number 5 is a so-called "hard" paper, object (a special cut-out of paper, a pencil, a designed to strengthen the contrast in very flat finger, or any object that can be made to fit negatives. The "average" negatives should along the shadow side margin of the image). print on number 2 or 3 paper. This is "dodging" of part of the negative. Con- versely, to bring out any detail still not blocked FINISHES. Omitting the "special effects" completely by the clumps of precipitated silver finishes and textures, which are unsuitable for in the dense highlight areas of the negative, the scientific work, the choice narrows to two com- light passing from the film image to the print- mon finishes -- glossy and matte. For prints ing paper can be allowed to fall on this area an that must be blackened around the edges of the extra amount of time by passing the rays from picture to blend into a black background, matte the film image to the printing paper through a finish will hold the blacking compound better hole in an opaque sheet of paper or cardboard, than glossy, which tends to flake off with hand- which shields off the remainder of the projected ling. Many photographers prefer glossy, how- image. This "burning in" brings the tone of the ever, because the image appears brighter. highlighted area closer to the rest of the image. WEIGHTS. I n w e i g h t or thickness, Such manipulations with the projected image printing papers may be half -weight, normal- must be done with artful care. The object used weight, or double -weight. Half -weight papers to dodge or burn should be kept well above the are very thin and crease easily. Although they paper (on which the image comes into focus) in are suitable for scientific illustration, they are order to keep the rays passing the edges of the seldom used because they are somewhat fragile object diffracted rather than sharp. In addition, and must be handled with more care than thick- the underexposure or overexposure of the areas er papers. Double -weight papers resemble in printing should be gradational, with the shad- postal cards in thickness, and are mostly used ow areas progressively underexposed from the for commercial portraits and exhibition prints. undoctored area to the edge of the specimen's Normal-weight or "single-weight1' paper is most image, and the highlights progressively over- readily available and fulfills the requirements of exposed from the undoctored areas to the center durability, strength, and easy processing. of highlight density. Sometimes a compromise can be reached PROCESSING ROUTINE between the need to emphasize details of low- Precautions. -- In processing either film relief structures and the desirability of a bal- or prints, certain precautions are worth noting. anced negative. It is always true that details that never registered in the shadow areas of the CLEANLINESS. Particularly films are lia- negative and details that are completely blocked ble to damage and ruin by sloppy techniques. out in the highlight areas of the negative can Stains spoil the image for printing and dirt can never be made visible in the print. Neverthe- scratch the emulsion or become embedded in it. less, careful and considerate dodging and burn- Even if a dust-free air circulation is not ing can extend the tonal range in problem shad- available, frequent cleaning and scrubbing of ow and highlight areas. In any case, the depar- all floors, walls, and equipment in the dark- ture from a balanced lighting should be the min- room will sooner or later pay dividends in av- imum required to shpw the features which would oiding negative and print damage. Exposing a 32 PAPERS ON PALEONTOLOGY few new negatives per day, after all camera Bulk photography. -- Numerous photomic- equipment has been dissembled and stored, can rography jobs will entail production of many involve much more time than keeping the dark- photographs at the same magnification. Speed room clean. in production and uniformity in results can be TEMPERATURE CONTROL. Developer, rinse expedited by the establishment of a routine. bath, and fixing solution should be at nearly the MAGNIFICATION. Adjust the camera to give same temperature. Passage from cold to hot a desired magnification on the film. If the cam- solutions and the reverse can cause strains in era is unadaptable and lacks a bellows for bring- the emulsion which may exceed its elastic limit. ing a specific magnification to the film, set the Sudden chilling and contraction may cause the camera to give maximum enlargement of the emulsion to crack, and sudden heating and ex- largest specimen in the group to be photograph- pansion may cause wrinkling or reticulation. ed; then photograph a millimeter scale (divided All processing solutions can be brought to about into hundredths, if such is available) on one room temperature before using by placing the frame of the roll. Maintain this setting through- trays containing the solutions in a water bath. out the photographic project. Such planning is better than trying to establish the magnification MAINTAINING SOLUTION PURITY. Splashing the solutions as the print or film is passed into them of each specimen later by comparing actual can have dire results. The developer can be measurements against the print. rendered useless by a few drops of the short LIGHTING AND NEGATIVE CONTRAST. Allow a stop bath. The hypo can be exhausted by dump- few short sections of film for determining the ing a number of unrinsed prints directly into it. best lighting of the specimen to show its essen- Contamination of solutions is to be avoided. tial characteristics, the best diaphragm setting Darkroom layout. -- Within the same space to get sufficient depth of field, the best exposure a convenient or very inconvenient layout can be time to give a negative of good balance, and the zrmged . If possible, the processing of iiim best developer to yield good contrast and dens- should be separated from the processing of the ity. Inspection of the first developed frames prints . may point up the need for modification of the set-up. When "perfected, " make a record of The lab space for film processing can in- all factors for future reference anduse. There clude the stored film (in bulk, if cartridges are is no need to repeat this phase of experimenta- to be loaded), work table for developing, and tion for each photographic job. lines for drying films. The lab space for paper Proceed to shoot all the specimens which processing can include the enlarger, storage of can be advantageously illustrated with this set- various grades of paper, work table for pro- cessing, and drying facilities. up. Postpone any special lighting or higher magnifications that may be indicated or desired. Proper selections of safelights should be Insofar as possible, treat each class of special installed in each room, together with normal problems in a new routine. Again, record the lights for inspection of results and for routine factors responsible for giving the best results. cleaning and other operations. The chemicals PRKESSING FIW. Even though several hun- used in making up the solutions, or the made -up dred frames of film are to be processed at one solutions, can be stored if necessary outside the time, show equal care in handling each roll. darkroom but close at hand. Cameras, lenses, Keep developing temperature constant. Be es and lights for shooting should never be stored - pecially careful not to scratch undried film. in the darkroom where they are apt to be affect- ed by chemicals used in processing. Note any deficient negatives and re-shoot before modifying the set-up. Proper ventilation of the darkroom is a must. Fumes should be promptly removed by PRINTING. Adjust the enlarger to give the forced circulation of the air. desired final enlargement on the printing paper, using the frame bearing the image of the milli- meter scale. Select an average negative and same contrast is produced by processing for 2 find the ideal lens opening and exposure time for minutes at 90°F as would be produced in the the enlarger to give a print which will develop same film (exposed the same time) and develop- in the recommended time for the paper-develop- er by processing for 10 minutes at 55°F. Of er combination (usually 1$ to 2 minutes). Ex- course, these extremes of temperature are not pose the first print of each frame for this time, recommended, and a preferable development and modify the time to give a print that will would be about 7 minutes at 6a°F. match the other pictures in contrast and density The manufacturer's recommendations for closely. Seldom will the first print be exactly each film should be heeded. Some are unduly right. Nevertheless, the production-line appro- influenced by high temperatures and react with ach to printing of the production-line negatives a variety of damaging and blemishing maladies. is a time-saver. Most films are designed for best results when developed at 68°F. REVISING AND IMPROVING THE NEGATIVE AGITATION.The movement of the solution Factors in processing an emulsion. -- It over the emulsion promotes devlopment as it is true that many factors affect processing: the brings a fresh supply of the chemicals into con- type of emulsion, the chemical composition of tact with the surface. For that reason, films the developing solution, time, temperature, and/or papers processed in a tray with agita- amount of dilution, and kind of agitation. The tion develop faster than films dipped into a tank emulsion has already been selected by this or allowed to rest in a tray. The effects of agi- stage, and the film manufacturers offer certain tation are very secondary to those produced by recommendations for the choice of developer. additional time or higher temperature. We can assume that such a choice has been de- cided before turning to the other factors. TIME-GAMMA-TEMPERATURE CHARTS. For films in particular, these charts supply the TIME. The length of processing time can photographer with information to plan his pro- be used to control the density and contrast of cessing and to improve his results. the exposed negative, within limits. As will be expressed in the section on "Negative Faults, " Gamma has already been discussed. It is however, no developing process can bring out a factor relating the density range in a correct- details in shadow areas if the rays from the sub- ly exposed negative (NDR) to the brightness ject did not result in any change in the halides range in the subject (SBR) NDR in that part of the negative; and no process can Gamma = bring out details in highlight areas if the light IgZE from the subject was so strong that the develop- The gamma achieved-in development can bring ment changes ail the silver halides to metallic out the full potential of the latent image in the silver throughout the area of the negative. Nev- film and make it printable. ertheless, prolonged development can make an With each commercial developer, the acceptable negative from what would be other- maker supplies time -gamma-temperature in- wise too flat; and shortened development can formation in the form of a chart for each of the make a good, printable negative from one that films for which the developer is likely to be would otherwise be over -contrasty. used. The time-gamma-temperature chart for Ideally, the time can be set within narrow the particular film-developer combination is a limits of the recommended time by changing the graph with temperature in OF or "C as the ord- eqxsure of the ne-tive or dtering the lighting lhate and time of development in minutes as the of the subject. abscissa; sloping subparallel lines across the graph are labeled with values of gamma pro- TEMPERATURE. One of the most often ignored duced by those temperatures and times. and yet one of the most influential factors in development is the temperature of the solution. As shown in the following table of gamma In one film and developer, for example, the adapted from the data of a typical time-gamma- i PAPERS ON PALEONTOLOGY

DATA FROM TYPICAL TIME-GAMMA-TEMPERATURE CHART

Values of Gamma

Developing Time (minutes)

2 3 4 5 6 7 8- 9 10 12 15 20 25 30

Values of Developing Time (minutes)

Gamma

Typical gamma values for grades of paper: Gamma Grade of contrast 1.10-1.35 NO. 1 .90-1.10 No. 2 .70-. 90 No. 3 .55-. 70 NO. 4 .40-. 55 NO. 5

the same gamma is reached subject brightness range (SBR). Next, he can at high temperature for a short time as at low select the negative density range (NDR) which temperature for a long developing time. would fit the printing paper he plans to use, and After a few experiments, the photograph- from the Table of Negative Density Ranges pre- er can decide approximately the value of his sented above learn what gamma of development will give this NDR. For example, from a few through-the -lens light meter, an immediate experimental exposures, he finds that his SBR decision can be made on the proper exposure. is about 24. To print his negatives on Number With microfossils, which cover only part of the 3 paper he desires a NDR of .80; the gamma of field of the film, however, the background is development should be about 0.6. Now, if the also registered on the meter. If the background time -gamma-temperature data given here ap- is appreciably darker or lighter than the micro- plies to his film type and developer, he should fossil, it is better to take the meter reading on process his films for 10 minutes at 65'F; this a light-gray card which closely matches the will produce the gamma of 0.6. overall tone of the specimen as coated. By means of gamma, the charts for the Negative faults. -- The contrast in the film film-developer combination in use and the Table can be corrected by changing (1) the subject of Negative Density Ranges provide interrela- lighting, (2) the development gamma, or (3) tions on the lighting of the subject, the develop- both. The lack of detail in the shadow or high- ment time, and temperature. They act as a light areas of the image can be corrected by guide in the search for the best negative that changing (1) the lighting of the subject to make can be produced with available equipment. it more nearly uniform and balanced, or (2) the exposure time. Printing on various grades of Proper exposure. -- The data on gamma is based on the assumption of proper exposure. paper can remedy contrast to a degree, but the The proper exposure time can be derived in negatives produced should rarely if ever require two ways. the hardest or softest contrast in printing paper. The following chart is useful in analyz- EXPERIMENTAL. The approach most widely ing the deficiencies of negatives. It serves employed in arriving at correct exposure for best in the experimental stage, as various the particular lighting and lens setting is to combinations are being tried to discover the expose a number of frames at variant times, ideal one for the series of photographs. It is develop all at an intermediate gamma, and sel- always better to correct the exposure, the ect the exposure time which produced the best lighting, or the processing than to try to rem- negative density range. edy incorrect negatives in printing. This is LIGHT METER. If the photomicrographer is especially true when numerous photographs fortunate enough to have access to an accurate are planned for a "standard" set-up.

TABLE OF NEGATIVE FAULTS

Fault Cause Prevention Remedy

More exposure, less Soft paper for excess- Contrasty, lacking Forcing development of development; more even ive contrast; no rem- shadow detail underexposed negative lighting of specimen edy for lack of shad- +J LQ at higher level ow detail m k +J Develop correct time r~0" Contrasty, full shad- Overdevelopment of and temperature; use normally exposed nega- Soft paper +J ow detail lower gamma; higher u tive a, angle lighting may k ~1 help 8 C Less exposure, shorter Soft paper for excess- H Contrasty, highlights Overexposure, overdev- development; weaker ive contrast; no rem- blocked elopment highlighting of speci- edy for lack of high- men at higher angle light detail TABLE OF NEGATIVE FAULTS (continued)

Fault Cause Prevention Remedy

Hard paper for excess- Flat, lacking shadow Underexposure, under- More exposure, incre- ive flatness; no rem- $ detail development ased development edy for lack of shadow a detail !- +J C 0 Underdevelopment of higher gamma in U development; lower Flat, full detail correctly exposed Hard paper angle lighting may 0 negative al help !- !- 8 Development cut too Ward paper for excess- Flat, highlights block- short trying to tor- Expose less, develop ive flatness; no rem- ed out rect overexposed nega- "omally edy for lack of high- t ive light detail Hard paper for flat- Dense, highlights Overexposure and un- Less exposure, full ness; no remedy for blocked, other tones derdevelopment trying development lack of highlight de- flat to correct tail Dense, highlights Overexposure, correct No remedy for lack of blocked, other tones development Decrease exposure correct highlight detail

Dense, highlights No remedy for lack of .rl Overexposure, over- Less exposure, less highlight detail; soft co blocked, other tones too contrasty development development paper for excessive $ contrast

+J U Thin, lacking shadow Hard paper for excess- ive flatness; no rem- detail, other tones Underexposure, under- Increase both exposure edy for lack of shad- flat development and development H ow detail Increase exposure; Thin, lacking shadow Underexposure, correct softer lighting at Normal paper; no rem- detail, other tones development higher level of speci- edy for lack of shad- correct men may help fill in ow detail shadows

Increase exposure, de- Soft paper for excess- Thin I lacking underexposure, over- crease development; ive contrast; no rem- detail, other tones softer lighting at development edy for lack of shad- too contrasty higher level of speci- ow detail men

anywhere between the outermost and innermost MULTIPLE LENSES lens elements. Very few lenses are now made which are All of the discussion of photography above molded equally biconvex of one kind of glass. applies to what is commonly known as a "single- What may appear as a "single" lens may be lens" camera (even though "single unit of lenses three or four lenses mounted in series or cem- mounted together" or "single set of lenses" ented together, and ordinarily two or more might be more appropriate. -glass comtmsitions are used in their manufact- ure. The bptical center of such lens series can Experiments have shown years ago that lie somewhere within the innermost lens or the problem of depth of field -- which concerns the photomicrographer in particular -- may be image rays and unwanted reflections. The per improved by using a far lens in the camera sys cent of reflected light is higher for glass with a tem of fairly long focal length stopped down (to higher index of refraction. give greater depth of field) and a near lens to Stray light in lens system. -- The light magnify the image transmitted by the far lens reflected from a lens surface creates two dis- onto the film. advantages: (1)the intensity of the light trans- Here a few of the factors that affect a mitted to form the image is diminished by this two- or three-lens camera will be discussed, amount, and (2) the reflected light may, by add- particularly as they apply to the use of a micro- itional reflections and refractions, reach the scope as the lens-bearing part of the camera. film and harm the image. The number of glass- As with the composition of the fixing solution, a air surfaces is an exponent of the useful trans- new ingredient (in this case, lens) is added to mitted light, so that each additional lens re- counteract a deficiency in the first, and itself duces the transmission more than did the pre- introduces new deficiencies which must be vious lens. This relationship could be anticip- counteracted or neutralized by still other add- ated, since each additional lens starts new re- itions. The addition of lenses to the camera flections from the rays which have already been does not lead necessarily to "perfect" pictures, weakened by reflections on and in the first lens. with "perfect1' resolution and "perfect" depth of field; instead, it is the best available comprom- ise between conflicting contributions to better USEFUL LIGHT TRANSMISSION images on the film. Let n = refractive index of glass in lens Light transmission of lens. -- Of the light TOT = total light transmitted through the passing through a single lens or a series of lens (TRL + RFL) TRL = useful ("true image") transmitted lenses, part will be refracted rays from the light subject being passed on to form the image on RFL = reflected light (per cent) the focal plane and part will be reflected, re- p = number of glass/air or air/glass sur- flected and refracted, reflected and refracted, faces in the system [Equation #31] etc. rays originating from reflections of the n - 12 RFL = (- ) If n = 1.5, RFL = 0.040 and original rays on the glass surfaces. The first n+l rays are the useful light transmitted by the lens if n = 1.7, RFL = 0.067; the 0.05. to the image of the subject, and the second rays mean RFL for a glass/air surface = TRL for one glass/air or air/glass surface = 0.95. are stray light that has reflected at least twice on lens surfaces before reaching the film, and there tending to destroy the image. The more No lens transmits 100% of the incoming (incident) lenses, the more reflections in the system. To light. Each additional lens in a system cuts a degree, the glass of which the lens is made transmission still more. With seven lenses, controls the division of the light rays into true- less than half of the light is transmitted.

Lenses P TOT TRL stray Stray (surfaces) -TRL

Some reflections set up on three glass- air surfaces by an incoming ray. - PAPERS ON PALEONTOLOGY

STRAY LIGHT ILLUMINATION ON THE FILM 1 - RFL Let 0 = angle between optical axis and emer- TOT = [Equation #33] 1 + RFL (p - 1) gent ray from the lens when focused B = brightness of subject E, = illumination on film 1 - RFL k = transmission of lens (ratio of light Stray = - - (1 - RFL)~ [~q#341 1 + RFL (p - 1) getting through the lens to the light falling on the lens Stray light becomes a serious problem when numer- E, = ks~sin2 Q [Equation #35] ous lenses are in a system. With as many as ten lenses, the stray light transmitted by reflections, refracted reflections, reflected reflections, etc., can be as strong as 36% of the "true image" light transmitted.

Illumination of field of image. -- The in- tensity of illumination on the film is always less than the light intensity falling on the lens, be- cause the glass comprising the lens loses a 1 ens film portion of the incoming light by reflections. In addition, as noted previously, the aperture, Angle 6' sin Q Illumination * which controls the area of the lens through which light can penetrate, has a direct effect on the illumination of the film. As shown below, the illumination on the film is directly propertional to the transmissi~n of the lens (which varies only slightly from lens to lens) and to the square of the sine of the angle formed at the film surface between the optical axis and a line through the outer limit of the cone of light emerging from the lens. The angle of this emergent ray is independent of the size of the lens or the angle at which the rays entered the lens. Illumination varies over the field of the * Illumination in foot-candles, where image regardless of the overall illumination, B is measured in candles/sq.ft. being concentrated at the optic axis and decrea- Only the angle of the emergent ray is a factor sing toward the edges of the field. If the angle in film illumination; the angle of the enter- formed from the diaphragm center to the corner ing ray is not a factor. of the film is very large, the difference between The illumination on the film is the same for illumination at the center of the film and illum- each f-stop, regardless of the distance of the ination at the corners of the field may be so subject: if the subject is only half as far great that every image appears to be centrally away, its brightness would be 4 times as great, bright and peripherally dark. Obviously, the but its image would be magnified 2 diameters longer focal length lens and the longer camera and the light would be spread over 4 times the will alleviate a serious imbalance in illumina- former area; hence, the illumination per unit area is constant. tion of the f.ield. No matter how many lenses are used in the camera system, the emergent ray from each lens con- trols the illumination passed on to the next lens, and SO on through each of the other lens- the lens without change and be condensed by re- es until the film is reached. fraction as it leaves the lens. By the time the VARIATION OF ILLUMINATION OVER THE FIELD OF THE beam reaches the concave side of the planocon- IMAGE cave lens it may cover an appreciably smaller circle. Then, upon entering the second lens, Let 0 = angle between line through the center the rays will be refracted back to their original of the aperture and a line from the center of the aperture to any point direction and continue on to the next lens in the on the film system. E = illumination at angle @ from the cen- ter of the aperture E, = illumination at center of field

If such a lens is reversed, the light beam entering the plane surface of the planoconcave lens will be expanded as it leaves the lens, with each ray diverted away from the optic axis. aperture film Those rays reaching the convex surface of the Table of Variation of Illumination planoconvex lens will again be made parallel. over Field of Image The beam through the small central area of the first lens can thus be expanded to cover the Angle @ from Relative illumination optical axis (light intensity) whole area of the second lens.

0O 1.000 5" 0.984 lo0 0.941 15O 0.870 20° 0.780 25O 0.675 40' 0.344 60' 0.062

Only the central part of the cone of emergent light from each lens in the system transmits Of course, the actual optics are more fairly even illumination over the field of complex than the simplified example given here, the image. but the principles are the same. The difficult- ...... ies of matching the optical properties of one Reversible lenses. -- Some lenses are lens against the other to have good resolution in now on the market which give one set of magni- both directions are so great that such lenses fications when mounted normally and a different not made by many of the world's leading lens set of magnifications when reversed (turned manufacturers . around, mounted "backwardsTv). The explana- Two-lens system. -- The addition of a tion of this "magic" lies in a simple principle second lens to the optical system of the camera of refraction between two lenses. If the lens may produce smaller, greater, or the same system contains within it a planoconvex lens magnification; it may make the focal length and a planoconcave lens with the same curva- longer or shorter, or leave it the same; it may ture, a light beam of parallel rays entering the improve the quality of the image or it may neg- plane side of the planoconvex lens will enter ate the ability of the lens system to form any 40 PAPERS ON PALEONTOLOGY

image at all. The optics of the two-lens system is pri- marily concerned with changing the focal length. Addition of the second lens does not necessarily enlarge the image produced by the first. &\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ TWO-LENS SYSTEM FOCAL LENGTHS

Let fl= focal length of first lens f2 = focal length of second lens focal length of combined lenses The equations above apply also to combinations $(1+2)= q = distance between optical centers of of positive (convex) and negative (concave) the two lenses lenses :

[Equation #37]

[Equation #38]

. . film JlJ2 [Equation #391 4 = fl + f.2 - c2, Example 5: Two lenses, one positive (convex) and [Equation $401 one negative (concave), are separzted by a short distance : f1 = 32 mm Example 1: Two lenses of short focal lengths -- f2 = -16 mm f = 16 mm and f = 32 mm -- are separated by 4 mm; what is their combined focal length? -16 x 32 - -(I6 32) = -42.7 mm. The '=32-16-4 12 lens system is negative, acting as a concave lens and Example 2: If the same two lenses are separated never focusing. by 44 mm, what is their combined focal length? Example 6: A positive lens (f = 16 mm) and a negative lens (f = -32 mm) are separated by 4 nun: -32 x 16 - - (32. x 16) - = 25.6 mm Example 3: If the same two lenses are separated '=16-32-4 -20 by 16 mm (the focal length of one of the two Example 7: The same two lenses (example 6) are lenses), what is the combined focal length? separated by 44 mm: l6 32 = 16 mm (the focal length of =16 32 16 + - the lens which equalled the separation distance) Example 8: It is desired to produce a focal length of 100 mm by adding a second lens at 50 Example 4: If the same two lenses are separated nun from a lens of 25 mm focal length. From equa- by 48 rmn (the sum of the two focal lengths of tion #40: the lenses), what is the combined focal length?

Example 9: Two lenses, focal lengths of 25 and (See the illustration at the top of the next 50 mm, are to be combined to produce a l&ns column. ) system with a focal length of 500 mm; how far apart should they be placed? A supplementary lens attached very close to the lens of a camera focused on subjects at infin- ity will change the focus of the camera to Example 10: A camera has a lens system with foc- subjects in a plane whose distance from the al length of 16 mm; what lens should be added camera lens is equal to the focal length of mm 32 from it to change the focal length to 10 the supplementary lens. For two psi tive (convex) lenses, increasing the distance between them lengthens the combined Example 11: A lens of focal length = 16 mm is focal length. combined with one of focal length = -8 mm from The limiting case for two positive lenses is which it is separated by 4 mm; what is the com- reached when the distance between them equals bined focal length? the sum of their focal lengths; at that point, -8 x 16 - -8 x 16 - - - -32 mm. The combin- their power becomes zero and the focal length =16-8-4 -4 ation acts as a is infinite. This is known as an afocaz sy~t@?l concave lens and does not reach focus. and is used in telescopes. Example 12: The two lenses of example 11 are For a positive (convex) and a negative (concave) separated by 32 mm; what is the combined focal lens, increasing the distance between them length? shortens the combined focal length. For a positive and a negative lens, the combina- tion will always achieve focus (be positive) if the numerical value of the negative focal. Two-lens System Summary length is equal or more than the numerical value of the positive focal length.

$1 f2 9 f (l+2) For a positive and a negative lens, the combina- tion will never focus (will always be negative and equivalent to a concave lens) if the num- + + 0 fl f2 /fX erical value of the positive focal length + + Short Short equals or exceeds the sum of the numerical + + value of the negative focal length plus the fl f 1 distance between the lenses -- fl 2 (f2+ 9). + + f2 f2 This situation may be described as one in + + Long Long which the lenses are separated by a distance equal to or less than the sum of the focal + + = fl + f2 Infinity + + >fl+f2 Nofocus f - 0 fl f2 /fX Three-lens system. -- Because in such a -+ <4-fl Focus system the innermost lens can only modify the + =4-fl Inf inity image as it has been transmitted by the two + H-fi No focus outer lenses, the equations for factors in a + =-fl Any Focus three-lens system are derived by considering first the focal length of the combined two outer + > f f, + f2 Infinity lenses and then the focal length of this two-lens + >-fl - > fl + f2 FOCUS soon becomes apparent that the direction of mounting the lens system can change the com- + <-fl Any Focus bined focal length of the three lenses. In the + - Short Long equations and examples below, "normal" will - be regarded as the position with lens #1 outer- + fl f 1 - most and "reversed" as the position with lens + f2 Impossible #3 outermost. + - Long Short For systems having more than three lens- 42 PAPERS ON PALEONTOLOGY es, formulas become even longer and more in- It is well to remember, in actual practice volved. However, the combined focal length of of computing focal lengths, that the opticaJ cen- a system with any number of lenses can be ter of a lens may not be the geometric center. solved by first computing the focal length of The main use of such computations in planning lenses #1 and #2, then computing for the #1+ #2 for a photomicrographic camera is to deter- combination and lens #3, then computing for the mine (1)whether the system will focus at all, #1+ #2 + #3 combination and lens #4, etc. , until and (2) the approximate magnification on film all lenses have been used in the computation. with the available bellows extension.

THFEE-LENS S'-STEM FOCAL LENGTHS Let fl = focal length of outermost lens (lens = distance between lens #1 and lens #l)in "normal" position of system (1+2) #2, regardless of position f2 = focal length of middle lens (lens q(2+3) = distance between lens #2 and lens #2) in the system #3, regardless of position f = focal length of innermost lens (lens 3 f(1+2+3) = combined focal length of the three #3) in "normal" position of system lenses For "Normal" Position of System

CCC [Equation #41]

[Equation #42]

[Equation #43]

,- -,

- fl (f 2 + f3) + f3 ($2 - q(1+2)) - flf2f3/f(1+2+3) (2+3) [Equation #44]

fl + f2 - q(,+2) CCC ,? 3 [Equation #45]

For "Reversed" Position of System

[Equation #461

[Equation #47]

[Equation #481

[Equation #49]

[Equation #501 Example 1: Three lenses of a camera have focal lengths (from outermost) : (1) 20 mm, (2) 30 mm, and (3) 40 mm. The first and second lenses are Hence, there are two positions to place the 10 mm apart and the second and third lenses are middle lens, each of which will give a combined 30 mm apart. What is the combined focal length? focal length of 60 mm! Using equation #41: Example 6: The first two lenses (see example 1), having focal lengths of 20 and 30 mm, will not focus when separated by 60 mm: Example 2: The lens system of example 1 is re- 20 x 30 --=- 600 -60 mm. versed. What is the new combined focal length f(1+2)= 20 + 30 - 60 -10 for the "reversed" position? Usins equation #46: Will these lenses as a system attain focus if they are placed 10 nun in front of a lens with focal length of 40 m?

Example 3: The lenses of example 1 are used in the "normal" sequence. The two inner lenses are - 24000 ---= 24000 80 mm. set 20 mm apart. Where should the outermost 1400 - 1200 + 100 300 lens (lens #1) be placed to give a combined focal length of 16 mm? Using equation #45: The three-lens system may be regarded as a two- lens system in front of one lens, tWt as a two- lens sytem behind one lens. Two lenses which together will not attain focus may, when combined with a third lens, form Example 4: With the same three lenses (as in part of a three-lens focusing system. example 1) arranged in the "normal" sequence, the two outer lenses are mounted as a unit 40 Reversal of a three-lens system rarely fails to mm apart. How far should this unit be mounted change the focal length of the system. in front of the rear lens to give a combined Any lens with a particular curvature on one side focal length of 240 mm? Using equation #44: and a different curvature on the other behaves optically as two lenses with little or no spac- ing between their optical centers. The equi- valent focal lengths of the two sides are difficult to determine without special instru- Example 5: With the same three lenses (as in ex- ments to measure the curvatures, and it is ample 1) in the '3normal" sequence, the outermost easier to find Ehe "combined" focal length in lens is mounted 100 mm from the rear lens. Where each direction by direct experiment. should the middle lens be placed to give a com- If the lens system is extended over an appreci- bined focal length of 60 mm? Using equation able interval, the addition of a concave lens #45, and letting will serve to shorten the combined focal leny- Q = q(2+3) and th, and conversely, the addition of a convex Q (1+2) = 100 - 9, lens will serve to lengthen the combined focal 24000 600 length. '(1+2)' loo- = ~O(Q- 40) + 20 + 30 - Q- - 40 If the lens system is condensed into a short in- terval, the shortest focal length is obtained by addition of a lens of short focal length. Q~ - 90Q + 1800 = 0 Similarly, the focal length can be increased fQ - 60) (Q - 30) = 0 by the addition of a lens of very long focal Q = 60, 30 length. Checking the combined focal lengths produced by qc2+3)= 60 and 30, we find: 24000 For q - (2+3)= 60, f = 20(70) + 40(-10) - 60(10)

24000 For q(2+3)= 30, f = 20(70) + 40(-40) - 30(-20) - PAPERS ON PALEONTOLOGY

regarded, in most cases, as rules handed down THE MICROSCOPE AS A LENS SYSTEM by the world's finest optical specialists. Of all the instruments involving high- Optics of the microscope. -- The object- grade optical systems, the microscope and the ive forms a real "aerialt' image at a certain camera are the two which have received the distance above the subject (specimen). When most attention in lens designs and the most re- this aerial image coincides with the focal plane f inement in engineering and manufacture. The of the eyepiece, the observer "sees" a virtual economics of the optical microscope, which is image projected somewhere below the level of used in high-school and university teaching and the subject. All modern microscopes have this in research, has led to its perfection. A con- eye-to-image distance fixed at 250mm. Now if tinuing market and competition between the the eye does not intercept the rays coming out leading manufacturers keep research and devel- from the eyepiece, they can be projected 250 opment thriving, and the optical systems of mm above the eye level position to form a real microscopes are in general superior to those image. Hence, a camera with film placed 250 in cameras. mm above the eye level position of the micro- The microscope can be adapted to provide scope will record the image of the specimen at the lens system for photomicrograrphy. With the magnification produced in the virtual image. the standardized design of current microscopes What was seen through the eyepiece becomes and their lenses, the adaptation for taking pic- what is recorded on the film. tures of microscopic specimens is much easier For the subject to remain in focus when now than at any time in the past. Many leading various objectives are used in the microscope, companies stock adapters and 35-mm backs to the distance from subject to aerial image must convert the microscope into a photomicrograph- be kept constant. Of course, the tube to which ic camera. lenses (both objective and eyepiece) are attach- Brands of microscopes. -- PJthcugh the ed needs also to rerna.in in the same psition design-of microscopes is essentially standard- and its length is fixed. Hence, when the sub- ized, each company has its own size of camera ject is in focus, the bottom of the tube (where body and its own sets of lenses. Parts are not the objective is attached) is a fixed distance regularly interchangeable between Zeiss, Wild, above the specimen; this is the so-called ffsub- Bausch & Lomb, American Optical, or other ject distance of the objective." The tube will makes of microscopes. Even though the differ- extend a short distance above the level of the ences in optics of objectives and eyepieces aerial image; this distance is called the "inter- (oculars) and in the length of the tube on which mediate image distance of eyepiece." The they are mounted may not be readily seen, the subject-to-image distance is the subject dis- tolerances in the combination are quite small tance of the objective plus the mechanical tube and extremely critical -- apart from the fact length minus the intermediate image distance that each company has its own dimensions and of the eyepiece. The figures given below are design of connectors and its own threading, for a Zeiss instrument. purposely made to differ so that their objectives Subject dis- Mechanical Intermediate Subject-to- and eyepieces will not fit onto thxciy of a tance of ob- tube image dist, image microscope made by a rival organization. Even jective length of eyepiece distance within the products of one company, each set of b + mtl - d - a objectives is made to be used only with a certain set of eyepieces. 45 + 160 - 10 = 195 rrna For the image to remain in focus when In designing a photomicrograph:TC camera eyepieces are exchanged, the eyepiece focal using a microscope, therefore, it must always plane for each eyepiece must always coincide be borne in mind that each system of lenses is with the plane of the aerial image. That is, finely precisioned to work in optical harmony. the eyepiece flange must also be located at a The manufacturer's recommendations should be fixed distance (intermediate image distance of NUMERTCAL APERTURE

Let d = apertural angle of objective lens (the angle formed when subject is in focus by intersection of optic axis with surface of subject (the center of the field in focus) and a line from this point to the edge of the aperture in aerial the objective image NA = numerical aperture of objective n = index of refraction of medium between subject and aperture of objective

NA = n sin d [Equation #51] The greater the NA value, the more light admitted into the microscope and the brighter the image. 'ncreasing the NA value has the same general eff- ect as opening the diaphragm of a camera lens (decreasing the f-stop number). Insofar as the microscope is being used as a camera lens, it gives less depth of field, For that reason, the ideal objective for photomicrography has a diaphragm. NIJMF,RICAL VALUES OF MAGNIFICATION AND FOCAL LENGTHS Let m,= numerical value of magnification of microscope m, = numerical value of magnification of objective (scale of image formed by objective) me = merical value of magnification of eyepiece (scale of image formed by eyepiece) f, = focal length of microscope lens system f,, = focal length of objective I I subject = focal length of eyepiece level f, * otl = optical tube length (not the same as mechanical tube length!) virtual mtl mechanical tube length; fixed by the ------_D------+---1- = image manufacturer to a specific length; the distance from the attaching mount the eyepiece) from the plane of the aerial im- of objective to the seating of the age in the tube; the eyepiece mounts must be eyepiece. so designed that their focal plane is always at a = distance from subject to aerial image d = distance from aerial image to seating a fixed distance from their seating. of eyepiece Hence, by careful designing of the mount b = distance from subject to attachment for the objective, its aerial image will occur of objective q = distance between optically equivalent at a fixed position in the tube; and by designing center points of objective and eye- of the mount for the eyepiece, its position of piece. focal plane can be made to exactly coincide with the aerial image. In this way, objectives By the construction of the microscope, when it is in focus : and eyepieces can be freely interchanged with- out re-focusing the microscope. 46 PAPERS ON PALEONTOLOGY

mtl+b-d=a [Equation #52] otl 250 250 250 mm=Tax~=~=- For each make of microscope, each of these fac- fe fm tors has a fixed value. otl

For the total system: [Equation #631 mm = 250/fm [Equation #53] Example 4: A 45x objective having focal length fm = 250/mm [Equation #54] of 4 nun is used with a lox eyepiece. What is fmmm = 250 mm [Equation #55] the focal length of the combination? The product of focal length x numerical value of From equation #58: otl = 45 x 4 = 180mm magnification in any standard microscope is From equation #60: fe = 250/10 = 25mm 250 mm. Thus, when in focus, the virtual im- 4x25 5 From equation #63: f = = -mm age seen in normal microscope use is 250mm m -180 9 below eye level; and when the image is pro- jected beyond (above) the eyepiece, it forms The same result could be obtained from the a real image (suitable for registration on numerical values of magnification (using film) at 250mm above eye level. equation #62) : For the objective:

mo = otl/fo [Equation #561

fo = otl/mo [Equation #57] The focal length of the microscope system is the fomo = otl [Equation #58] product of the focal length of the objective x focal length of the eyepiece divided by the Example 1: A 20x objective with a focal length optical tube length. of 8.3mm produces an optical tube length of: In the optical system of the microscope, q, the 20 x 8.3 = 163mm separation of objective and eyepiece equival- The optical tube length is the product of the ent centers, is always negative. Thus: focal length of the objective x its numerical -q = otl - f, - fe (see sketch) fEquation #641 value of magnification. From this we can derive equation #63 from #38: For the eyepiece: me = 250/fe [Equation #59] fe = 250/me [Equation #601 -q = otl - fo - fe fe me = 250mm [Equation #611 fo fe - fo fe = -- Example 2: A lox eyepiece has a focal length of: f, otl fo +fe + otl - $0 - fe 250/10 = 25mm Example 5: fo = 16 mm mo = lox fe = 25mm me = lox x The product of focal length of the eyepiece otl = 16 x 10 = 160mm the numerical value of its magnification is constant, fixed in all standard microscopes From equation #64: at 250mm. -q = 160 - 16 - 25 = 119nun For the system! = [Equation #621 m, mo x me The same answer is obtained from equation #63: Example 3: A 20x objective used with a 7,5x eyepiece produces a magnification of

Or the same answer can be got from equation #62: The numerical value of magnification of a micro- scope is the product of the numerical values of magnification of its objective and eyepiece. Combining equations #62, #56, #59, and #53: PAPERS 6N PALEONTOLOGY

Table of Microscope Magnifications

Eyepieces Hyper- Objectives Huygenian plane

12.5 fe 20x me--- --

1.9 97x 184 485 727 970 1212 1455 1.8 100~1 180 1 500 750 1000 1250 1500 I I Compensating

Numerical Aperture. -- The equation for for the objective with the greatest NA. NA is given above (equation #51). In designing and m&ing objectives, NA is of great concern. Theoretical Resolving Power of Objectives * Each manufacturer strives for the maximum NA possible without introducing serious aberr - Wavelength in microns ations in the marginal region of the lens. Red Orange Yellow Green Blue Violet Theoretically, the ability of the objective NA .700 .615 .590 .545 .485 -430 to resolve detail depends upon its NA and the 845 915 1020 1165 wavelength of the light used: .25 715 815 -50 1430 1625 1695 1830 2040 2325 lines resolved = 2 x NA/ wavelength of light .75 2145 2440 2540 2750 3060 3490 This is the theoretical or potential resolving 3255 3390 3665 4080 4650 power of the objective; how closely it is actual- 1.00 2860 ly approached depends upon the skill and pre- 1.25 3570 4065 4235 4580 5100 5810 cision in the lens manufacture. As shown in 1.50 4285 4880 5085 5500 6120 6975 the table below, resolving power is greatest for the short wavelengths (blue and violet) and * Lines per nun 48 PAPERS ON PALEONTOLOGY

Resolving power is not the only factor such as blue and red, are made equal. Other related to NA. Inasmuch as the numerical ap- colors fall near these two near-end members erture is a function of the actual apertural op- of the spectrum. These lenses are suitable ening and the objective-to-subject distance, it only for transmitted light; they are not for acts in part like an actual diaphragm in a cam- photomicrography. They are chromatically era lens. Thus the brightness of the image is corrected for two colors and spherically for one directly proportional to the square of the NA. color. Such objectives are designed for visual work, and are best with yellow-green -- the par- Similarly, just as increasing the diameter ticular color most effective with the eye. Be- of the opening in a camera lens will decrease cause these lenses are not fully corrected, they the depth of field, so increasing the NA value are not intended to be used with the usual eye- will also decrease the depth of field. It also pieces of higher than 12.5~magnification. Achro- decreases the flatness of the field by introduc- matic objectives are used with low-power (5x to ing aberrations in the increased size of the 12.5~)eyepieces of Huyghenian design or with effective portion of the lenses in the objective. higher-power (20x) eyepieces of Hyperplane de- Effects Produced by Increased MA sign. The combination of two kinds of glass in Factor Effect the achromatic lenses alleviates some of the problems of color differences in the image, but Resolving power Proportional increase it does not eliminate them. Even with the best Brightness of image Proportional to square correction, the achromatic objective retains of NA problems with residual colors of the secondary Depth of field Decrease spectrum. Flatness of field Decrease (2) APOCHROMATIC -- refined lenses of highest ~rdercorrect& to eliminate abber2ticns cbzom= It soon becomes apparent that using the atically for three or more colors and spherical- microscope objective as part of a camera lens ly for two colors. The colors of the secondary system counteracts the goals of the optical spectrum are eliminated completely in a good physicists who design microscopes. For nor - apochromatic lens, and only faint suggestions mal microscopy, the best lens has the highest of tertiary spectrum remain as residual color. NA (other factors being equal), with highest Such lenses are best suited for photography. resolving power and highest brightness of image They are used with compensating eyepieces to produced. To achieve this, depth of field and correct differences in magnification produced flatness of field have been sacrificed. This is by the objective. one of the ways in which photomicrography goes against the basic principles and premises of (3) SEMI-APOCHROMATIC -- improved over plain good microscopy. achromatic lenses, but not as fully corrected as apochromatic. Kinds of lenses. -- Uncorrected lenses (4) P~CHROMATIC-- improved chromatic cor - have many of the inherent defects or aberra- rection, and elimination of curvature of field. tions listed below, which make them unsuitable (5) PLANAPOCH~MATIC -- focal distances made for good microscopy and especially unsuitable equal for several colors. Some are made with for photomicrography. very high NA. Basically, there are two types of correc- (6) MICROTESSAR or LUMINAR -- lenses specially ted lenses: achromatic and apochromatic. But made for photomicrography without an eyepiece. there are also varieties according to the de- The shorter focal lengths (16-, 25-, and even gree of correction and just what has been cor- 40-mm) in these lenses may be combined with rected. specially designed eyepieces which optically (1) ACHROMATIC -- focal distances of two colors, complement their corrections. Lens aberrations. -- No lens is perfect. low, and orange between. By using two kinds The aberrations mentioned here apply to regul- of glass, as in an achromatic lens, an attempt ar photographic lenses as we1 as to microscope is made to bring two colors to unite at a com- lenses used in photomicrography. In the micro- mon focus. When this is done, the intermediate scope, the chief characteristics of the image colors fall closer to the lens than the common are controlled by the objective and the eyepiece focus and the extreme colors fall beyond. In serves mainly to enlarge the image so produced apochromatic lenses, three colors are brought and project it onto the film. Hence, the "defect" to a common focus, or as nearly as possible to of the objective is of prime importance. it. All determinations are made for a point source of light at a fixed distance out from the (1) SPHERICAL ABERRATION -- longitudinal varia- lens on the optic axis. tion of image position for different zones of the lens. Rays from a subject on the optic axis en- (4) CHROMATIC DIFFERENCE OF SPHERICAL ABERR- ter the lens at numerous pints and are refract- ATION -- different patterns of longitudinal vari- ed to cross the optic axis (on the image side) at ation of image position for different zones of different points, usually at successive points as the lens with different colors. The spherical the rays pass through more distant parts of the aberration (see item 1 above) may not be the lens. In uncorrected lenses, the rays which same for one color as for another. With tk pass through the outer parts of the lens fall combination of different glass compositions short of the focal point of rays passing through used to produce achromatic or apochromatic the lens near its center. In corrected lenses, lenses, one color may have a slight zonal aber- rays passing through the margin of the lens are ration whereas another color may have very made to refract at the focal point of the rays strong spherical aberration. The perfect lens passing near the center, but rays through part would have all parts of the lens reach a common of the intermediate zone may fall short of the focus for all colors, but such a lens has never focal point; this is called zonal aberration. The been produced. The best lenses reduce the de- lens can be tested for forms of spherical aberr- gree of spherical aberration for two or three ation by examining the image of a bright point selected colors. source focused at the center of the field. With (5) CHROMATIC DIFFERENCE OF MAGNIFICATION -- the lens closed down, the image is brought into variation of image size for different colors. sharp bright dot (tiny circle); then, with the This is not the same as longitudinal chromatic lens open and part (including the center) shield- aberration, in which the position of the focal ed, see if the dot still has good definition. point varies in different colors at the center of (2) COMA -- variation of image size for differ- meld. This concerns the different image ent zones of the lens. Coma practically ceases sizes produced by different colors, and mani- to exist near the axis of even an average lens, fests itseif by colored fringes around the edges but it increases steadily for images toward the of the image -- becoming stronger near the cor- edge of the field. The rays from a bright point, ners of the field. These fringes register on entering the lens at an angle from the optic ax- black-and-white film as blurred outlines. The is, are refracted in such a way that those defect is not serious in modern microscope through the outer parts of the lens form a cir- objectives, but it is not decreased in any way by cular image, but those through the rest of the stopping down the lens. lens are refracted less and fall beyond the (6) DISTORTION -- variation of magnification in circle (farther from the center of the field) to different parts of the field. If distortion is pre- produce a comatic (cometlike) form of image. sent, the magnification is not constant over the (3) LONGITUDINAL CHROMATIC VARIATION -- longi- field; the outer parts may be magnified more or tudinal variation of image position for different less than the central part. If the periphery is colors. In an uncorrected lens, the focal point magnified less than the center (as is obvious in varies for different colors: violet is the short- a picture of a divided scale extending across the est and red the longest, with blue, green, yel- field), the image of a square will have convex sides and appear "barrel-shaped. " On the other 50 PAPERS ON PALEONTOLOGY

hand, if the periphery is magnified more than better results are obtained in modern instrum- the center, the image of the square will have ents by a high-power objective and a low-power concave sides and assume the so-called "pin- eyepiece than by a low-power objective and a cushion" shape. Distortion is not affected by high-power eyepiece. This is true because the stopping down the lens. Lenses with distortion high-power objectives have relatively higher reduced to a minimum are called rectilinear or NA values and consequently brighter images orthoscopic lenses. and higher resolution. For photography, how- ever, the reverse is true. Better results are (7) ASTIGMATISM -- a longitudinal separation be- obtained with a low-power objective and a high- tween the images of radial and tangential lines power eyepiece, because such a combination in the field. This defect does not exist on the yields greater depth of field -- and the reduced axis of a well-centered lens, but may increase brightness of image is no appreciable handicap rapidly in oblique rays and be manifest near the in exposure of the film. edges of the field. If a wheel is photographed with an astigmatic lens, the radial spokes may Most makes of microscopes, unfortunate- be in focus while the tangential rim is blurred, ly, have few suitable objectives. The manufac- or vice versa. turers have devoted little effort to development of widefield low-power objectives with diaphr- (8) CURVATURE OF FIELD -- curvature of the "field agms. Perhaps this situation prevails because surfaces" obtained by joining up the radial and many photographic uses of the microscope in- transverse astigmatic images over the entire volve thin sections (slides) illuminated by light field. If all the tangential focal lines (circles) transmitted from below and presenting a mini- are joined from a plane subject (perpendicular mum of depth (thickness). to the optic axis, of course), they will reach their best definition on a curved surface called One combination which is manufactured the "tangential field curve" of the particular by Leitz includes the P1 Plano (achromatic) lens, Similarly, the radial focal lines all lie on lx objective (NA = 0.04; focal length = 33 mm; a "radial field curve" or the so-called "sagittal free working distance = 30 mm) provided with a field curve" of the lens. These two fields coin- diaphragm and the GF Periplan Widefield eye- cide at the center of the field, since all astig- pieces available in lox, 12.5x, 16x, and 25x. matism vanishes there. Stopping down the lens With these basic lenses, the Leitz microscope does not affect the positions of these curved can be equipped with micro camera attachment planes, but because it increases the depth of giving 1/3x or 1/2x with a focusing telescope field and shortens the focal lines themselves, to show the precise focus and coverage on the for practical purposes it reduces the effect of film in a Leica camera body. Thus, with P1 astigmatism on the image. lx objective, GF lox eyepiece, and 1/3x at- Common Lens Aberrations tachment, the image on film will be 1 x 10 x 1/3 = 3.33~;or with the same objective, GF Near optic axis Near edge of field 25x eyepiece, and 1/2x adapter, the image on film will be 1 x 25 x 1/2 = 12.5~.Intermedi- 1 Spherical aberra- 1 Coma ate combinations are also possible (without tion 2 Astigmatism re-focusing the microscope): 2 Coma 3 Curvature of field Magnifications on Film 3 Longitudinal chrom- using P1 lx Objective 4 Distortion atic aberration 5 Chromatic difference Eyepiece 1/3x Adapter 1/2x Adapter 4 Chromatic difference of magnification of spherical aber- ration

Combined microscope lens system. -- If the microscope is to be used normally (to magnify specimens for visual examination), PAPERS OF4 PALEOMTOLOGY 51

k la topographic, am vues st6r6oscopiques REFERENCES 5 1 p. , illus. , Par is, Gauthier -Villars . (Interesting exposition on the potential of illus. Adams, Ansel, 1970, Basic photo: 299 p. , the pin-hole camera. ) Morgan & Morgan. Crabtree, J. I., & G. E. Matthews, 1938, Pho- Allen, R. M. , 1958, Photomicrography, 2d ed. : tographic chemicals and solutions: 360 p., 441 p. , illus. , VanNostrand. illus. , Amer . Photographic Publ. Co. Angerer, Ernst von, 1953, Wissenschaftliche Croy, 0. R., 1969, The complete art of print- Photographie, eine Einfiihrung in Theorie ing and enlarging: 2 51 p. , illus. , Phila. , und Praxis: 227 p., 112 text-figs., Leip- Chilton Book Co. zig Akad. Verlagsgesell. Geest & Portig. Duffin, G. F. , 1966, Photographic emulsion Asher, Harry, 1968, Photographic principles chemistry: 239 p., 2 pls., illus,, Focal and practices: 288 p., illus., Phila., Press. Chilton Book Co. Dutton, Laurence, 1937, Perfect print control: Baines, Harry, 1967, The science of photogra- 160 p. , illus. , N. Y. , The Galleon Press. phy, 2d ed. : 318 p. , illus. , Wiley. Eastman Kodak Company (in addition to small Blair, J. M., 1945, Practical and theoretical pamphlets on films, papers, chemicals, photography: 243 p., illus., Pitman Publ. etc. ): Co. 1928 - Elementary photographic chemistry. Blaker, A. A., 1965, Photography for scient- 1940 - Kodak reference handbook; materials, ific publication: 158 p. , 20 pls. , 34 figs. , processes, technique. W. H. Freeman & Co. 1944 - Photomicrography; an introduction to photomicrography with the microscope, Boni, Albert, 1962, Photographic literature; an 14th ed. international bibliographic guide to gener - 1951 - Kodak graphic arts handbook. a1 and specialized literature on photogra- 1952 - Kodak professional handbook; mater- phic processes, techniques, theory, chem- ials, processes, techniques. istry, physics, apparatus, materials & 1957 - Photography through the microscope applications. . . : 335 p., Morgan & Morgan (Publ. P-2). ----- 1972, Photographic literature 1960-1970: 1959 - Kodak master photoguide. 535 p., Morgan & Morgan. 1967 - Kodak plates and films for science and industry (Publ. P-9). Boucher, P. E. , 1963, Fundamentals of photo- graphy, 4th ed. : 535 p., illus., VanNos- Feininger, Andreas, 197 5, Successful photo - trand. graphy: 320 p., illus., Prentice-Hall. Brain, E. B., 1969, Techniques in photography: Fritsche, Kurt, 1968, Faults in photography: 186 p., 35 pls., illus., Edinburgh, Oliver causes and correctives (transl. by L. A. & Boyd. Mannheim from "Das Grosse Fotofehler - buch"): 337 p. , illus. , N. Y. , Focal Carroll, B. H., D, Hubbard, & C. M. Kretsch- Press. man, 1968, The photographic emulsion: 276 p., 2 pls., illus., Focal Press. ~lafkidgs,Pierre, 1958, Photographic chem- istry (transl. from French by K. M. Clerc, L. P., 1954, Photography, theory and Hornsby): Fountain Press. practice (transl. of "La technique photo- graphique"), 3d ed. : 606 p., illus., Pit- Greenleaf, A. R. , 1941, Chemistry for photo- man Publ. Co. graphers: 177 p., illus., Amer. Photo- graphic Publ. Co. Colson, ~en6,1887, La photographie sans ob- jectif. Applications aux vues panoramiques Henney, Keith, & Beverly Dudley, 1939, Hand- 52 PAPERS ON PALEONTOLOGY

book of photography: 87 1 p. , figs. , tables, and professional, covering the field of McGraw-Hill. miniature camera photography: illus., Humphreys, W. J. , 1964, Physics of the air: N. Y., Morgan & Lester. 676 p., illus., Dover Publ. Co. Neblette, C. B., 1962, Photography: its mater- ials and processes, 6th ed. : 508 p., illus., Jacobson, C. I., & L. A. Mannheim, 1969, Van Nostrand. Enlarging, 20th ed., 534 p., illus., N. Y., Amphoto. ----- 1970, Fundamentals of photography: 351 p., illus. VanNostrand reinhold. James, T. H., & G. C. Higgins, 1960, Funda- , mentals of photographic theory, 2d ed. : ----- & L. D. Stroebel, 1948, An introduction 345 p., illus., Morgan & Morgan. to photography: 210 p. , illus. , Rochester Jordan, F. I. , 1945, Photographic enlarging, Instit. Tech. 3d ed.: 252 p., illus., Boston, Amer. Photographer's Handbook: 64 p., illus., N. Y., Photographic Publ. Co. Time-Life Books. Katz, Ernst, 1941, Contribution to the under- Roebuck, J. R., & H. C. Staehle, 1942, Phot- standing of latent image formation in ography, its science and practice: 283 p., photography: 107 p., illus., Utrecht, illus., D. Appleton-Century Co. Drukerij F. Schotanus & Jens. Shillabar, C. P., 1944, Photomicrography in Langford, M. J., 1965, Basic photography; a theory and practice: 773 p., illus., J. primer for professionals: 374 p. , illus. , Wiley & Sons. Focal Press. Snelling, H, H., 1970, The history and practice Lawson, D. F. , 1961, The technique of photo- of the art of photography: 139 p. , illus. , micrography: 256 p., illus., McMillan. Morgan & Morgan. ----- 1972, Photomicrography: 494 p. , illus., Sussman, Aaron, 1973, The amateur photogra- N. Y., Academic Press. pher's handbook, 8th rev. ed. : 562 p., Lester, H. M., & J. S. Carroll, 1957, Photo- illus., N. Y. , Crowell. lab-index, 17th ed. : 24 chapters (loose- S'wedlund, Charles, 1974, Photography; a hand- leaf in binder), illus., Morgan & Morgan. book of history, materials, and process- Loveland, R. P., 1970, Photomicrography; a es: 368 p., illus., Holt, Rinehart, & comprehensive treatise: 2 vols. , illus. , Winston. N. Y., Wiley. Time-Life Books, 1970, Light and film: 227 p., Mack, J. E., & M. J. Martin, 1939, The pho- illus. , New York. tographic process, 1st ed.: 586 p., illus., Vestal, David, 1975, The craft of photography, McGraw -Hill. 1st ed. : 364 p., illus., Harper & Row. Mason, L. F. A., 1966, Photographic process- Wall, E. J., & F. I. Jordan, 1947, Photograph- ing chemistry: 321 p., 4 pls., illus., ic facts and formulas: 364 p. , illus., Am- Focal Press. er. Photographic Publ. Co. Mees, C. E. K., ed., 1966, The theory of the Windisch, Hans, 1956, The manual of modern photographic process, 3d ed. : 591 p., photography (transl. by F. W. Frerk from illus. , McMillan. "Die neue Foto-Schule, die Technik"): 291 Monpillard, F. , 1926, Macrophotographie et p., illus., Phila., Rayelle Publ. microphotographie: 671 p., 86 text-figs., Paris, G. Doie & Cie. All references listed are in the library of The University of Michigan, although they are Morgan, W. D., & H. M. Lester, 1935, The distributed in the main library, undergradu- Leica manual; a manual for the amateur ate library, and branch libraries.

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