EXPOSURE METERS

By Jim Branch & Nasir J. Zaidi, ASC Member (Associate)

The usual final adjustment of a motion-picture camera for exposure control is made with the iris diaphragm in the camera lens. While this is a very simple adjustment, a great deal depends upon its accuracy. Much thought has gone into the objectives to be attained by the adjustment of the diaphragm, and the means to obtain a correct adjustment.

It is recognized that a prime object of exposure control in motion-picture photography is to obtain consistent and uniform images of the principal subjects. It is very important to obtain flesh tones which will be consistent from one scene to the next. It is undesirable to have flesh tones which will be light in one scene, dark in the next without reason, and again light in the next scene. Correct exposure control will provide negatives which are consistent from scene to scene and can be printed on a very narrow range of printer lights.

Modern exposure control is based on the use of a good light meter. The light meter measures the effective intensity of the light, taking into account the sensitivity of the film in the camera and the exposure time. The exposure time is a result of the frames-per-second rate at which the camera operates, and the angle of the shutter opening. Professional cinematographers usually think in terms of 24 frames per second and a 175-degree shutter, which give a basic exposure time of 1/50 second. The light meter combines all of the foregoing factors to give an answer in terms of the appropriate camera lens stop.

Never in the history of motion picture photography has the need for accurate exposure control been as great. With the increasing use of digital video postproduction to manipulate original motion picture photography, the cinematographer more than ever needs to monitor how the exposures generated on the original negative as densities will reproduce as digital electronic signals. Maintaining control of creative photographic visions can be greatly enhanced with the aid of a highly accurate exposure meter. The meter should be able to predict how original scene luminance will ultimately reproduce as print densities empowers the cinematographer to confidently explore maximum creative possibilities.

Light meters are of two types. Some measure the incident light which illuminates the subject. Others measure the light which is reflected from the scene. The results obtained from the two different types may be quite different. It is important therefore to understand the differences between the two types.

Incident Light Meters

These meters are normally used at the location of the photographic subject. They measure the light which is effective in illuminating the subject. They give an answer in terms of f-stop or T-stop for the camera lens. The camera lens diaphragm opening is then set to match the effective intensity of the prevailing illumination.

When the film is exposed, the various reflectances presented by the subject will then each fall into a given place in the film acceptance range. For example, a face tone of 30% reflectance will fall into the 30% reflectance position in the film acceptance range. This method thus provides consistently uniform face tones from scene to scene.

The incident light meter accomplishes its purpose by doing two things. It measures the incident light intensity at the location of the photographic subject. It also takes into account the conditions of illumination geometry; that is, whether the subject has front key light, side key light, or a back key light. The meter combines these factors and gives an answer in terms of the correct setting for the camera lens diaphragm. There are several makes of incident light meters which use a three-dimensional light collector. The hemispherical light collector allows these meters to perform automatically the dual function described above.

These incident light meters are normally used at the position of the principal subject, with the hemisphere pointed at the camera lens. The hemisphere then acts as the miniature face of the subject. All illumination which will be effective on the subject, including key light, fill light, line light, hair light, eye lights, etc., will be received, evaluated and integrated by the meter. The meter will then indicate directly the correct f-stop or T-stop for the camera lens. Incident light meters are particularly useful because they may be used on a scene before the principal subject appears. They may also be carried through a scene, with the hemisphere always pointed at the camera lens, to detect uneven illumination, and particularly hot spots, into which the subject may move during the action. This allows the scene illumination to be suitably balanced before the principal subject is at hand.

In the case of outdoor photography, it is not always necessary to take the meter to the location of the principal subject. Under such conditions the illumination is usually uniform over considerable areas. If the illumination is the same at subject location and at camera location the meter may be used at camera location. Care should be exercised to point the meter in the proper direction, as though it were at the subject location.

Exposure meters, in general, are either analog (with a needle) or digital. The introduction of the analog incident meter with the 3-D light-collecting hemisphere revolutionized the method of determining proper exposure for the cinematographer.

Today, a number of companies throughout the world manufacture exposure meters employing the basic incident type principles in their design, but all due credit should be given for the invention to Don Norwood, ASC, who patented it, and Karl Freund, ASC, who was instrumental in the development and design of the original “Classic” Spectra light meter. Most incident meters are provided with suitable adapters so that they may be converted for use as a reflected light meter if the occasion should so indicate. The reflected light adapter can be used in a situation where the cinematographer encounters difficulty in putting the meter into a position to read either the illumination directly on the subject, or illumination similar to that on the subject. Such a situation, for example, might be encountered when taking a picture out of the window of an airliner in flight. The reflected light attachment can also be used in other situations to evaluate the relative brightness of a background.

Special Effects

When a special effect is desired, the cinematographer may use the incident light meter to first determine normal exposure for the subject. Then he may then deliberately modify that value, up or down, to achieve the desired effect. This can be done with considerable confidence because the incident light meter will give a firm foundation upon which to base the desired modification.

Specific Situations

There are some situations, occasionally encountered in outdoor photography, which require special attention.

1. Unusually light or dark backgrounds are cause for consideration. When a scene includes an unusually light background, the cinematographer may wish to first use the meter as an incident light meter to determine the basic exposure for the principal subject in the foreground. Then he can convert the meter to a reflected light meter in order to measure the brightness of the unusual background. The second reading is then used to modify somewhat the basic incident light reading. The same procedure could be followed in the case of an unusually dark background. 2. Outdoor scenes that include a subject in the foreground as well as distant objects, such as mountains, in the background, usually also include considerable aerial haze, which may be invisible or only partly visible to the eye, but strongly visible to the camera. A frequent photographic result is a recording of the aerial haze overlaid on the scene background. This would give the appearance of an overexposed background. It is recommended that in such a situation a haze-cutting filter be used to improve the background. In addition, use the procedure previously described for the case of an unusual lighting background.

3. Scenes consisting of a mixture of sunshine and shade areas, with the principal subject in a shade area, can be handled by: (a) using the meter in the sunshine area, or (b) opening up the lens by 1/2 to 2/3 f-stop from the meter indication.

Reflected Light Meters

Reflected light meters can be classified into two groups, according to function. The meters in each group may give exposure readings which are substantially different from those given by the meters in either of the other two groups. This is due to differences in basic principle of operation.

Group 1. These are the meters which are designed to measure the average brightness of an entire scene. Such meters are usually used at camera location and pointed at the scene. For a discriminating observer, this method appears to give acceptable results only in the case of a very limited category of scenes, those which have front-lighting and a foreground subject of medium tone as well as a back- ground of medium tone. In other types of scenes, which include side-lighting or backlighting, or very bright or dark backgrounds, or large areas of sky, the exposure results are questionable. This is because the meter, when used by this method, is affected not only by the unit brightness of each portion of the scene, but also by the relative area of each. Thus a large area of sky would influence the meter to dictate a small lens aperture which might result in an underexposure of the face of the principal subject in the foreground. Any backlight may strike directly into the meter cell and cause an unduly high reading on the meter. This also would result in underexposure of the foreground subject. Large bright backgrounds tend to cause meter readings which result in underexposure of foreground subjects.

Large dark backgrounds tend to cause meter readings which result in overexposure of the foreground subject. If this method is used it should be considered only as a very rough guide, subject to considerable modification according to the experience of the cameraman.

It is interesting to note that this method is the one generally used in the built-in automatic exposure control systems of amateur motion-picture and still picture cameras. It has been noted by many that the photographic results do not meet the high standards of professional cinematography.

Group 2: These are the spot meters. A spot meter may be used at camera location and aimed at a selected spot in the scene. The effectiveness of the meter is heavily dependent on the operator's judgment in the selection of the spot. The selected spot must be precisely representative of the particular combination of elements, which compose the scene. In the use of such a meter the operator must be particularly careful when confronted with a scene that presents strong contrasts between the selected spot and the scene background. An example of such a situation would be a case where a person in the foreground is in front of a very light background, such as sky or white buildings, etc. In such a case the operator should modify the spot reading provided by the meter according to his own estimate of the situation. When the use of a reflected light meter is required, the results of determining the exposure can be greatly improved by using a "Kodak Neutral Test Card."

This card is a piece of sturdy 8"X 10 "cardboard that is neutral gray on one side and white on the other. The gray side reflects 18% of the light falling on it, and the white side reflects approximately 90%. Also, the gray side has a protective lacquer overcoat that reduces specular reflectance and resists damage due to fading, fingerprints, soil, etc. To a light meter, an average scene is one in which the tones when averaged form a tone brightness that is equivalent to middle gray -- a tone that reflects 18% of the light illuminating it (the same tone and refiectance of the gray card). When a scene is not average the gray card as a reference helps you make the proper exposure judgments. A Kodak Gray Card is manufactured under close tolerances to provide a neutral gray-side reflectance of 18% ( +/- 1% ) and white-side reflectance of approximately 90%.

Testing

Small errors may exist in meters, lens calibrations, emulsion speeds and development. These small errors will frequently cancel out without undue harm to the final picture. It is when these errors add up in the same direction that their cumulative effect is serious. It is wise, therefore, to test equipment, film and meters under simulated production conditions so that errors may be detected and corrected before production begins. It is always a good idea to "tune up to the variables."

Exposure Meters- Incident Type

The Specifications of the exposure meters listed below are the meters designed exclusively for the cinematographers and therefore have included functions such as f/stop, footcandles/lux which are necessary for Cine photography.

Spectra Professional IV-A

Type: Spectra Professional IV-A Advanced Digital Exposure Meter (ADEMTM) for measuring Incident and Reflected light. Reflected light acceptance angle is 42 degrees-square.

Technology: For highest accuracy and reliability it utilizes Advanced front end op amp, Custom Sealed Hybrid electronic circuitry, Multiple Range Linear Circuit (MRLC) controlled by Advanced microcomputer and custom liquid crystal display with Backlit Electroluminescent lamp.

Light Sensor: Silicon Photovaltaic cell , Computer selected glass filters tailored to spectral response of the film. Swivel head assembly turns 270 degrees.

Measuring Capability: Direct readout of photographic exposures. Measures contrast ratios and average in f-stop and illuminance modes. Measures Illuminance level in footcandles (fc)and lux (lx).

Measuring Range: One million to one (20f/stop) direct reading Multiple Range Linear Circuit.

Digital f-stop: f.35 to f128 in 1/10-stop increments.

Analog f-stop: f0.7 to f45 in 1/3-stop increments.

Photographic Illuminance: Footcandles 0.1 to 70,000 and lux 1 to 100,000.

Contrast Ratios: Calculates ratio of key plus fill to fill light alone from 1:1 to 0.2 999:1. f/stop- difference: Plus or Minus 12 f/stop. Average: Average of two readings and or continuous averaging 0.3 in f-stop and footcandle mode.

Display Range: ISO: 3 to 8000 in 1/3-stop increments. FRAMES PER SECOND: 2 to 360 (for motion picture cameras). Exposure time: 1/8000sec to 30 minutes.

Resolution: Digital 0.1-stop. Analog: 0.2-stop

Accuracy: Digital 0.05 f-stop. Calibration is traceable to National Institute Of Standard Technology (NIST), Washington, DC. Additional Functions: Memory Recall switch is to recall stored reading in memory (m1), current reading in memory (m2) and /or the Ratio, Average and f/stop difference of the two readings.

Instant Recall Of All Measurement And Calculations: By pressing the recall (RCL) switch, the first reading (M1), the second stored/memorized reading (M2, contrast ratio or brightness (f/stop) difference, average footcandles or average f/stop, can be recalled.

Immediate Update Of All Exposure Calculations: If the film speed (ISO), frames per second (FPS) or exposure time settings are changed, the f/ stop are immediately recalculated and displayed in normal, f/stop difference, or average modes.

Memory Erasure: You can erase the stored/memorized readings in memory (M1) and memory (M2) by pressing the store (STR) and recall (RCL) switch simultaneously.

Camera Shutter Angle: Complete list of shutter angles and equivalent compensation table conveniently provided in the instructional material.

Bright Green Display: Electroluminescent lamp for viewing displayed measurement at very low light level.

Power Consumption: Operating 5 mA/reading. Data Retention 5uA.

Power Source: One 6 volt battery no's: A544, or PX28L, or PX28.

Estimated Battery Life: Approximately one year with normal use.

Dimensions: 5-1/2 X 2-1/2 X 2. Weight: Approximately 6 ounces

Accessories Supplied: Spectra meters are supplied with Photosphere (for incident light), Photodisc (for key or directional light measurement), Leather carrying case, Neckstrap, Battery, instructional manual and warranty card.

Warranty: Two year limited warranty. Specifications subject to change without notice.

Spectra Professional IV

Type: Hand-held exposure meter for measuring incident and reflected light.

Light Sensor: Silicon Photovoltaic cell, computer selected glass filters tailored to spectral response of the film. Swivel head 270 degrees.

Measuring Capability: Direct readout of photographic exposures. Also measures illuminance level in footcandles and Lux.

Measuring Range: One million to one (20 f-stops) direct-reading multiple-range linear circuit controlled by microcomputer.

Display Range: ISO film speed: 3 to 8000 in 1/3 stop increments.

Camera speed: 2 to 360 frames per second.

Resolution: Digital: 0.1 f-stop. Analog: 0.2 f-stops.

Accuracy: Digital: 0.05 f-stop. Additional Functions: Memory store and recall.

Lamp: Electroluminescent lamp for backlit liquid crystal display.

Power Consumption: Operating (reading) 5mA. Data retention 5uA.

Power Source: One 6-volt battery. (A544, PX28L or PX28).

Estimated Battery Life: Approximately 1 year with normal use.

Dimensions: 51/2"X 21/2"X 2". Weight: Approximately 6 ounces.

Reflected Light Meters (Spotmeters)

The Specification of the Reflected Light Meters listed below are of the One Degree Spot measuring type.

Pentax One degree spot meter Measuring System: Spot measuring of reflected light. Exposure read- out: LED digital display of EV Numbers with two dots (each of which equals +1/3 EV).

Photosensitive Cell: Silicon Photo Diode.

Measuring Angle: 1° (small circle in the viewfinder)

Measuring Range: (ASA/ISO 100) EV 1 ~ 20 (EV 20 displayed as “O”)

Scales: ASA/ISO 6 ~ 6400; Aperture f/1 ~128; Shutter speed, 1/4000 sec. ~1 sec. ~ 4 min.; Cine Index (24 frames/s); IRE,1 ~ 10.

Measuring Distance: From about 1.5m to infinity, fixed focal length (focusing down to about 1m possible screwing the eyepiece out as far as it will go).

Viewfinder: Pentaprism type with unreversed, upright image. Viewfinder coverage similar in shape to that of a 35mm SLR, and a diagonal field of view of 26°, horizontal field of 17°, and vertical field of 14°, viewfinder magnification of 1.15x, adjustable eyepiece of –2 to +1 diopter.

Power Source: One 6V silver oxide battery or equivalent alkaline battery.

Battery Check: failure of LED digital display to illuminate indicates battery needs replacing.

Others: Equipped with Tripod Socket and 40.5mm filter thread.

Dimensions & Weight: 45.5mm (W) x 144mm (H) x 83mm (D). 250 g (without battery).

Accessories: Case, wrist strap battery and lens Cap.

Minolta Spotmeter F

Type: Spot-reading reflex-viewing exposure meter for ambient and flash light.

Measuring method: Reflected light by silicon photocell detector masked for 1° angle of acceptance. Optical system: Through-the-lens reflex type utilizing semi-silvered mirror and pentaprism: Focus fixed for readings 1.3m to infinity, with optional close-up lens 0.6 to 1.4m.

Field of view: 12x17 with 1° area marked by circle in finder.

Magnification: 1.4 x

Measuring range (at ISO 100): Ambient: EV 1.0 to 22.5

Flash: f/ 2 to 90+0.9stop

Display range: ISO 12 to 6400 in 1/3-stop increments

Shutter speed (ambient): 30 min to 1/8000 sec. In 1-stop increments (cine: 1/50 sec.)

F number: f/0.7 to 90+ 0.9 stop in 1/10-stop increments

EV numbers: -4.3 +28.5 in 1/10-stop increments

Brightness difference: -9.9 to +9.9 stops in 1/10-stop increments f-number, EV number, and brightness difference shown in both external and finder displays. Analog scale (f-number): 1.4 to 45 in ½-stop increments.

Other indication/displays: Analog and digital display readouts change automatically to reflect ISO/time-input Changes; “S”, “A” or “H” on internal display indicates exposure is calculated for shadows, average (midtones), or highlight areas respectively “star” mark appears when reading on digital display is found for taking brightness- difference measurements; flash mark appears when using flesh mode.

Brightness difference indication: When “star” mark appears in external/ finder digital displays, difference in brightness between calculated measurement and subsequent readings is shown in 1/10- stop increments; calculated reading displayed again when measuring button is released.

Other functions: Memory: 2 channels, both indicated by pointers on analog display: digital recall possible; measuring level adjustment screw: tripod socket.

Repeatability: 0.1 EV

Power source: One AA battery

Operation Temperature Range: -10 to 50° C (14 to 122° F)

Storage Temperature Range: -20 to55°C (-4 to 131°F)

Dimensions (W x H x D) : 48x150x89mm (1-7/8 x 5-7/8 x 3-9/16in.)

Weight: 240g. (8.5 oz.) without battery

Motion Picture Screen Brightness/ Luminance spotmeters are included in this section simply for the benefit of the cinematographers who are interested in the quality of the projected print.

Spectra Cinespot One Degree Spot Meter

Type: Through-the-lens viewing spot-reading automatic/manual luminance meter.

Light Sensor: Silicon Photovoltaic cell with 1° angle of acceptance.

Viewing Optics: 1.6X magnification, erect system with focusing eyepiece.

Measuring Capability: Direct readout of luminance level in foot lamberts or candelas.

Measuring Range: Low Range 0-30 FL (or 0-100 cd/ m2) readings legible down to 0.5FL. High Range 0-300 FL (or 0-1,000 cd/m2), upper limit may be increased by use of accessory 10X or 100X attenuators.

Spectral Response: Within +/- 4% (by area) of CIE Photopic Luminosity Function.

Accuracy: +/- 1% of full scale or +/- 5% of reading (whichever is greater).

Error Due To Chopped Light: +/- 0.5 % at 24 cycles/ second.

Power Source: One 6-volt battery. (Eveready 544 or equivalent).

Estimated Battery Life: Approximately 1 year with normal use.

Dimensions: 5" X 2"X 6.4" Weight: 15 ounces.

Minolta Luminance One degree Spotmeter

Type: Reflex-viewing spot-reading automatic/manual luminance meter.

Light Sensor: Silicon Photovoltaic cell with 1° (1/3° in model nt-1/3°) of acceptance.

Viewing System: Focusing through-the-lens reflex type. Objective lens 85mm f/2.8. Angle of view: Circular 9° with central 1° (1/3° in model nt-1/3°) marked circle. Magnification: 2.96X focused at infinity.

Measuring Capability: Direct readout of luminance level in footlamberts or candelas.

Measuring Range: Model ft-1°: 0.01 to 99900 ft-L (0.01 step) Model nt-1°: 0.1 to 99900 cd/m2 (0.1 step) Model nt-1/3°: 1.0 to 99900 cd/m2 (0.1 step)

Display Range: Red (+) LED's at the right of the number display indicates 10X and 100X the display reading.

Accuracy: Within +/- 4% of C.I.E. standard +/- 1 digit in last display position.

Screen-flicker accuracy: Within 1% of average luminance with projection cycle of more than 72 Hz and duty of 7% (projector at 24 fps).

Analog Output: Output voltage: 1V over full scale. Output impedance: 10 kilo-ohms.

Power Consumption: 6 mA in analog mode. Meter can monitor changes in luminance for a period up to 40 hours. Power Source: One 9-volt battery (Eveready 216 or equivalent).

Estimated Battery Life: Approximately 1 year with normal use.

Dimensions: 2 7/8" X 6 3/8" X 4 11/16"

Weight: 18 1/8 ounces, without battery.

Much of the material in this section of the manual is basic, but reference should be made to Don Norwood, ASC, Karl Freund, ASC and Eastman Kodak Company for the gray card information.