Imaging October 23, 2008 Authors: Miguel Alonso, The Institute of Optics Joe Malach, Corning Tropel Andy Murnan, Genawave Paul Murphy, QED Technologies

Revised By: Jonathan Petruccelli, The Institute of Optics Christopher Todd, The Institute of Optics Write-up

Write-up ...... Overview ...... 1 Background ...... 4 Imaging Demonstrations...... Pinhole camera ...... 8 Flashlight illumination as a sum of images...... 13 Lens camera...... 15 Forming an image with a concave mirror ...... 21 Forming an image with a soda bottle…………………………………………………………23 Single lens imaging Demonstration ...... 27 Optical system demonstration…………………………………………………………………… Anatomy of a camera ...... 36

Overview

The basic idea An optical image is a replication of an object using light, where the replication retains the proportions of the original object. Imaging is the process of forming an image.

Dictionary definition Im age \ ‘im-ij\ n : An optically formed duplicate, counterpart, or other representative reproduction of an object, especially an optical reproduction formed by a lens or mirror.

Source: The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company.

1 To form an image, light emitted by or scattered from an object must be selectively allowed to pass through a pinhole or collected and focused by a lens(es) or mirror(s). An obstacle with a pinhole forms an image by blocking all light rays except for one (or a very thin bundle of them) from each point of an object. Therefore, each point on a screen placed behind the pinhole receives only one ray, coming from a corresponding point of the object, as shown in Figure 1.

Most light rays are blocked, but a pinhole allows one light ray to pass from each point on the light bulb An image of the light bulb is Light bulb emits light formed on a screen in all directions

Figure 1: Pinhole forms image

A lens, on the other hand, forms an image by collecting a bundle of light rays from every point on the object and focusing them to corresponding points on the screen. This is shown in Figure 2, where the gray area represents a bundle of rays collected and focused by the lens from one point on the light bulb to the corresponding point on the screen.

An image of the light bulb is Light b ulb emits light Lens collects rays from each point of formed on a screen in all directions the light bulb and images each point onto a screen.

Figure 2: Lens forms image

Definitions Ray : Line along which the energy carried by light travels.

Focusing : The action of making a bundle of rays from a point on the object converge to a corresponding point in the image.

2 Lens : a piece of glass, plastic, or other transparent material with smooth sides (at least one of which is curved) that is used to change the direction of rays. Lenses that are thicker at the center than at the edges have the capacity to focus light rays, and therefore to form real images. These are called converging or positive lenses. Lenses that are thicker at the edges cannot form real images on their own, but can be used in combination with converging lenses to make cameras of better quality. They are called diverging or negative lenses. Lenses work by refracting light at each of their surfaces. See Figure 3. Refraction follows Snell’s law. θ2 θ3

θ4 θ1 n1 n2 n3

R1 R2

Figure 3: Lens showing a single ray refracting at both surfaces. The ray, coming from a medium with index of refraction of n 1, enters a lens whose material has index n 2. The ray hits the lens’s first surface at an angle of θ incidence of 1 with respect to the surface’s normal. This surface, with radius R 1, refracts the ray at an angle θ θ 2 . The ray then travels to the second surface, hitting it at an angle 3 . Finally, the ray leaves the lens at an θ angle 4 from the surface normal and enters a medium with index of n 3. See refraction demonstration for more information. Real image : an image that can be projected onto a screen and is accessible. See Figure 4 Top.

Virtual image : an inaccessible image that appears to come from a particular point behind a lens or mirror, but in fact comes from an object at another location. See Figure 4 Bottom.

Object

Real Image

Object Virtual Image

Figure 4: Top: Lens forms a real image of the object. Bottom: Lens forms a virtual image of the object.

3 Background

History ~500 BC The earliest mention of the pinhole camera was by the Chinese philosopher Mo-Ti. He formally recorded the creation of an inverted image (an image that is rotated by 180 degrees) formed by light rays passing through a pinhole into a darkened room.

~350 BC Aristotle (Greece, 384-322 BC) understood the optical principle of the camera obscura. He viewed the crescent shape of a partially eclipsed sun projected on the ground through the holes in a sieve, and the gaps between leaves of a plane tree.

~300 BC Euclid (Alexandria). In his Optica he noted that light travels in straight lines and described the law of reflection.

965-1020 Ibn-al-Haitham (Basra, Iraq) used spherical and parabolic mirrors and was aware of spherical aberration (a lens or mirror defect in imaging). He also investigated the magnification produced by lenses and atmospheric refraction. He gave a full account of the principle of the pinhole camera including experiments with five lanterns outside a room with a small hole.

1490 Leonardo Da Vinci (Italy) gave two clear descriptions of the pinhole camera in his notebooks. Many of the first camera obscuras were large rooms like that illustrated by the Dutch scientist Reinerus Gemma-Frisius in 1544 for use in observing a solar eclipse.

1558 Giovanni Battista Della Porta (Italy), in his book Magiae Naturalis , recommended the use of a camera with a converging lens as a drawing aid for artists.

1604 Johannes Kepler (Germany). In his book Ad Vitellionem Paralipomena , Kepler explained vision as a consequence of the formation of an image on the retina by the lens in the eye and correctly described the causes of long-sightedness and short- sightedness.

1608 Hans Lippershey (Netherlands) constructed a telescope with a converging objective lens and a diverging eye lens.

1609 Galileo Galilei (Italy) constructed his own version of Lippershey's telescope and started to use it for astronomical observations.

1611 Johannes Kepler (Germany). In his Dioptrice , Kepler presented an explanation of the principles involved in the convergent/divergent lens microscopes and telescopes.

4 1647 B Cavalieri (Italy) derived a relationship between the radii of curvature of the surfaces of a thin lens and its focal length.

1668 Isaac Newton (England). As a solution to the problem of chromatic aberration (image defects related to color) exhibited by refracting telescopes, Newton constructed the first reflecting telescope using mirrors as he believed that chromatic aberration could never be eliminated by lenses.

1733 Chester More Hall constructed an achromatic compound lens using components made from glasses with different refractive indices.

1873 Ernst Abbe (Germany) presented a detailed theory of image formation in the microscope.

1953 Frits Zernike (Netherlands) won the Nobel Prize in Physics for his invention of the phase contrast optical microscope.

1990 The Hubble space telescope was positioned in a low Earth orbit on 25th April.

Everyday examples of images At the movie theater, film from a movie reel is imaged onto the large screen for all to see. A camera, film or digital, is a typical example of an instrument that forms images of objects. The image is recorded on film or an electronic detector. You see the result of this image in a photograph or the digital image on a display. The most familiar imaging instrument is the eye. When you look at a car on the street, an image of the car, as well as the rest of the scene, forms on the back of your eye. Your eye is a biological lens and a detector, and is constantly forming images.

Applications / engineering Standard applications of imaging are eyeglasses, magnifying glasses, camera lenses, telescopes, and microscopes. All of these applications are also taken to the extreme. Instead of eyeglasses, the cornea of the eye can have custom correction with LASIK surgery using a laser. Professional camera lenses are nearly fully automated and have excellent resolution. The Hubble telescope circles the earth in space to avoid problems caused by the atmospheric absorption of certain wavelengths of light and turbulence. There are optical microscopes that can see objects embedded in human tissue at shallow depths.

An example of a magnifying glass is shown in Figure 5. A positive lens can magnify an object when the object is close enough to the lens that it is within the lens’s focal length.

5

Virtual

Image Object focal l ength of lens

Figure 5: Simple magnifying glass forms a magnified virtual image of the object.

A telescope works by magnifying the range of angles of the rays from a distant object entering the objective of the telescope compared to the angles of the rays leaving the ocular. This is to say that if there were no telescope, the angles of rays entering your eyes would be the same angles of the rays entering the objective of the telescope and there is no magnification of the image in your eye. With the telescope, the rays leave the telescope at larger angles and therefore provide a larger image on the retina in the back of your eye. Objective Ocular Real Image

focal length of Objective focal length of Ocular

Figure 6: Keplerian Telescope. The angle of rays entering the objective is small compared to the angle of rays leaving the ocular and this provides the magnification for the eye. The magnification is also the ratio of the objective focal length to the ocular focal length. A magnified view of a distant object is formed on the back of the eye when the viewer looks through the telescope and the image appears inverted.

Ray Diagrams

To trace rays through a “thin” lens you can follow two or three rays through the lens (see figure 7). A ray that leaves an object and traveles parallel to the optical axis is refracted by the lens through the rear focus F’(Ray 1), which is at a distance from the lens of f, also known as the focal length of the lens. A ray that heads towards the center of a “thin” lens is unchanged in its direction (Ray 2). A ray that passes through the front focus F is refracted parallel to the optical axis (Ray 3). Notice that when the object is moved closer to the front focus F the distance to the image increases and the image size becomes larger. The reverse is also true.

Object 1 Optical F’ 2 Image Axis F 3 3 2 1 f f

6

Object 1 Optical F’ Image Axis 2 F 1 3 2 f f 3 Figure 7

7 Imaging Demonstrations Pinhole camera (also known as stenoscope or camera obscura)

Materials Needed - Can with clear (semitransparent, translucent) plastic lid. (Wegmans brand coffee or peanuts are good and cheap. The coffee comes also in big cans.) - Black or dark colored cardboard - Scissors - Scotch tape - Small nail and hammer - Aluminum foil - Pin

Procedure 1. With a small nail, make a hole at the center of the bottom of the can about 1 to 2mm in diameter.

8 2. Remove lid. Cut a strip of dark cardboard to fit inside the can. Fix it inside, so it covers the inner wall, in order to prevent light hitting these walls from reflecting onto the screen.

cardboard

3. Put the lid back on.

4. Tape a piece of cardboard around the can, forming a cylinder of about two or three times the length of the can, such that the bottom of the can is at the bottom of the cylinder. Add tape joining the can’s bottom to the cardboard, to block any gap. The cardboard helps preventing any outside illumination from interfering with the viewing of the image.

tape

9 5. Cut the upper end of the cylinder, so that it fits your face comfortably, without letting much light in.

6. Point your camera towards a bright object (through a window on a sunny day, a lamp, etc.) and you will see an inverted image (object is rotated 180 degrees) projected in the plastic lid.

10 7. The image can be viewed better if you block any light that might be entering through the cardboard. You can, for example, cover the cardboard tube with aluminum foil.

8. If the image is not bright enough, you can make the hole a bit bigger. This will cause, however, the image to become blurrier. If you want to make the hole smaller to reduce the blurriness, you can tape a piece of aluminum foil in front of the hole, and then with a pin make a smaller hole.

11 Explanation All points of the objects that we can see emit or scatter light in many directions. This light travels in straight lines called rays. What the pinhole camera does is to select only one “ray” from each part of the object. These rays go through the hole, and then hit the plastic lid, which works as a screen, forming an upside-down image. This image can be hard to see because of the small amount of light coming through the hole.

Useful information For more information on the history of the pinhole camera and directions for building more sophisticated models, see the following websites: http://brightbytes.com/cosite/cohome.html http://www.pinhole.org//make/build.cfm http://www.howstuffworks.com/question131.htm http://www.exploratorium.edu/light_walk/camera_todo.html http://www.kodak.com/global/en/consumer/education/lessonPlans/pinholeCamera/ http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/pinhole-camera/

12 Flashlight illumination as a sum of images

Materials Needed - Flashlight with smooth (not segmented) reflector - Piece of cardboard with small hole

Procedure 1. Illuminate the wall or a screen with your flashlight. The bright spot will look something like this:

13 2. This bright spot is really many overlapping images of the filament inside the bulb. To “single out” one of these images, place a piece of cardboard with a small hole in front of the lamp, so that the hole is not centered but towards one side. You will then see on the wall or screen a projection of the filament:

3. If you move the hole around, the perspective and size of the filament will change, giving the illusion of a filament rotating in 3D . The total bright spot of the lamp is just a superposition (or sum) of all these images (i.e. perspectives) of the filament.

Explanation Again, we have the pinhole camera effect. This time, the image is formed by rays that leave the filament, reflect at the reflector, and go through the hole towards the wall. When you change the position of the hole, the apparent point of view of the filament changes.

14 Lens camera (and projector)

Materials Needed - Converging lens with focal length of about 10cm. - Can with clear (semitransparent) plastic lid. (Wegmans brand peanuts 10 oz, for example.) - Two yogurt containers (Size 32 oz) - Scissors - Can opener - Scotch tape - A second plastic lid (or white plastic bag), for projector - Bright flashlight, for projector

Procedure 1. With the can opener, remove the bottom of the can.

2. Cut also the bottom of one of the yogurt containers.

15 3. Push the upside-down can firmly into the top of this yogurt container.

4. Make a hole at the center of the bottom of the second yogurt container. This hole should be a bit smaller than the lens.

5. Cover the sides and bottom of this outside container with dark cardboard or aluminum foil.

16 6. Tape or glue the lens at the bottom of the container, centered at the hole.

7. Insert the yogurt container with the can into the one with the lens.

8. The camera is ready. You can focus objects at different distances by sliding the inner container in and out of the outer one. The image is projected onto the plastic lid. Because the lens collects much more light than the pinhole, the images are much brighter.

17 9. You can also use your camera as a projector. Pull the inside container out, and stick a slide to the plastic lid. Put this container back inside the other one.

10. Put a second translucent plastic lid (or a few layers of white plastic bag) on top of a flashlight, in order to make the illumination diffuse.

18 11. Place the flashlight next to the viewer of the camera.

12. Turn the lamp on, and project the image onto a wall or screen in a dark room. Again, you can focus the inverted image by moving the second container with respect to the first one.

19 Explanation Lenses bend light, due to refraction. Converging lenses can collect many rays coming from a point on a bright object and bend them such that they cross at a point on a screen, forming an image. Because many rays are used instead of just “one” (that is, very few) as in the case of the pinhole camera, the images are much brighter and easy to see. The diagrams below show a comparison of the pinhole camera and the lens camera. The image in the second case is brighter than the first one by as many times as the area of the lens is larger than that of the pinhole.

20 Forming an image with a concave mirror

Materials Needed - Small concave (magnifying) make-up mirror, available at any drugstore or supermarket for about $3. These mirrors usually have two sides, one flat and one concave. Only the concave side works.

- White paper or white wall. - Window, TV or bright computer monitor.

Procedure 1. Hold the mirror in front of both the window (or monitor), with the concave side facing it. Move it until you form an image of the outside (or the image in the monitor) onto the wall or a piece of paper.

21 Explanation A concave mirror, like a converging lens, can bend rays to make them converge at a point, as shown in the diagram.

22 Forming an image with a soda bottle Equipment -2 liter soda bottle -Scissors -Water -Overhead lighting -White paper

Procedure 1. Use scissors to cut a roughly circular piece of the bottle from just below the neck. You should be cutting where the bottle is curved both towards the neck and around the bottle, so that the piece you cut out forms a shallow bowl shape.

2. Place a white piece of paper on the floor (somewhere where you won’t mind spilling a little water).

23 3. Pour a little water into the bowl-shape and slowly move it towards the piece of paper. Stop when it is about 15 cm (roughly 6 inches) from the paper. You should see an image of the overhead light projected on the paper. If you’re using an incandescent light like I was, you’ll see a bright spot which is a slightly-blurred image of the light bulb.

4. Now, get a small object, such as a pen, and place it on the piece of paper. Instead trying to form an image of the overhead light, try looking through the bowl at the pen. You should notice that the bowl is acting much like a magnifying glass.

Explanation Recall the definition of a lens from the introduction. It has at least one curved side, and in order to form real images (such as the light on the paper) it must be thicker at the center than at the edges. When you first cut out the bowl-shape it was curved, but the plastic was equally

24 thick in the center as at the edges. However, when you poured water into the bowl, the depth of the water was thickest in the center and thinner towards, the edges, which allows the bowl-with- water to bend light rays and act like a lens. The technical term for such a lens is plano-convex, since the top side is a flat plane and the bottom is curved outwards (convex).

When you focus the light from the ceiling onto the paper, the lens is bending the light rays from each point on the light bulb so that they all arrive at the same point on the piece of paper, as illustrated here.

25 When you instead look through the lens at an object, the lens bends the light rays so that they seem to be coming from a much larger object than they actually are. This object seems to be on the other side of the lens, and it is what is called a virtual image.

More information

This demonstration was adapted from : http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/make-a-camera-from-a- lemonade-bottle/ http://en.wikipedia.org/wiki/Concave_lens

26 Single lens imaging Demonstration

Equipment Lens (small plastic convex, obtained from OSA Educators’ Day or another small size positive lens) Flashlight Clear plastic diffuse bag (Ex. Wegmans plastic grocery bag) Scissors Scotch tape Poster tack or double sided tape Folder Styrofoam Cups (Quantity 1 or more, Size 20oz) Permanent markers of various colors Yardstick or meterstick Binder Clips (2 large) Graph Paper (1 sheet) Ruler

Set up and assembly 1. Tape yardstick down to table using Scotch tape leaving one long side untaped to provide an optical rail.

2. Look through the lens at an arms length out a window and make sure that the image you see is inverted. Take the plastic lens and use poster tack or double sided tape to position it on the table, next to the ruler, as demonstrated in Figure 1. (If your flashlight is taller than the lens, you may have to prop the lens up on something such as a notebook in order to form an image properly). Ensure that the orientation of the lens is square to the table and optical rail by hand. If the image is not inverted please find a shorter positive focal length lens. Plastic Lens

Poster tack

Taped down meter or yardstick

Figure 1

27 3. Cut the bottom out of another 20oz Styrofoam cup. Cut a 4” diameter circle out of a colorless diffuse plastic bag. Tape the plastic over the bottomless cup such that the plastic is taut and there are no wrinkles in the plastic over the hole. Using various color permanent markers, draw a graphic or graphics onto the plastic. See Figure 2. These graphics become the object that is imaged by the lens. Measure and note the size of the graphics with the ruler.

Various colored graphics on plastic stretched over bottomless Styrofoam cup.

Figure 2

4. Cut another 4” diameter circle out of a colorless diffuse plastic bag again. Tape the plastic over the front of the flashlight such that the plastic is taut and there are no wrinkles in the plastic over the flashlight. Place flashlight next to the optical rail so that it shines through the lens. Then place the cup with graphics over the end of the flashlight. Place the flashlight/cup assembly on the table, next to the optical rail, and center the flashlight and cups with the lens. See Figure 3. Position the cup with graphics about 305mm (or 12”) from desired image plane (say at beginning of optical rail).

Figure 3

5. Measure grid spacing on graph paper with ruler and note spacing. Fold sheet of graph paper in half to make the paper roughly 8.5” by 5.5”, or tape it to a piece of thick paper. Clip two large binder clips on either edge of the graph paper and position assembly at the desired

28 image plane (say at beginning of optical rail) and position the paper roughly centered on the axis of the lens.

Demonstration: Single Lens Imaging with a fixed object to screen distance 1. Turn flashlight on and adjust the position of the lens against the optical rail and slide lens next to your object (cup with graphics). Then move the lens away from the object, ensuring that the lens slides against the optical rail, until a sharp image is formed. See Figure 4a. Count the number of grid squares for the portion of the image. This is the first image with the lens at the first position. Compare the image size to the object size. Please note that if the first image is out of focus or is never formed then increase the distance between the object and the screen by approximately 100 - 200mm or (4 -8”) and repeat this step.

Figure 4a

29 2. Continue to move the lens away from the object until a second sharp image is obtained. See Figure 4b. Note the size of this image by counting the grid squares and compare that to the object. This is the second image with the lens at the second position.

Figure 4b

Follow up discussion/questions

- Why are the images inverted?

- What are the magnifications of the first image at the first lens position and the second image at the second lens position? Are the magnifications the inverse of one another? Discuss some ideas of why this may possibly be the case?

- If an unknown focal length lens is used: What is the focal length of the lens?

- If the object to screen distance is narrowed until the [object to screen] = [4 times the focal length of lens] and the demonstration repeated, why does eventually only one image form?

30 Also why when the object to screen distance is narrow enough ([object to screen distance] < [4 times the focal length of lens]) no images are formed?

Deeper explanation: An interesting characteristic of using a thin lens of focal length f to project an object on to a screen at a fixed distance d from the object is that there are two possible positions for the projection lens to form real images when d ≥ 4f. It can also be shown that these two possible positions have images that have inverse magnifications from one another. If the focal length of the lens is unknown, by measuring the size of the object and one of the images, to determine the magnification, and measuring the object to lens distance, the focal length of the lens can be found. Figure 5 and Figure 6 illustrate a thin lens at the two positions that produce sharp images.

Lens Position 1

Object

Image 1

S1 o S1 i d (fixed distance)

Figure 5: Projection lens of focal length f at position 1 where S1 o is the distance from the object to the thin lens at position 1 and S1 i is the distance from the lens at position 1 to the image and d is the sum of S1 o and S1 i.

Lens Position 2

Object Image 2

S2 o S2 i

d (fixed distance)

Figure 6: Projection lens at position 2 where The distances S2 o and S2 i correspond to the to the object and image distances from the lens at position 2 respectively, and the sum of S2 o and S2 i is d.

To find the distances involved in forming an image with a thin lens we can begin with the lensmaker’s formula:

31 1 1 1 + = (1) So Si f

where the So is the object to lens distance and Si is the lens to image distance and are both positive. This is rearranged:

S f S = i (2) o − Si f

The sum of So and Si are equal to the fixed object to screen (image) distance d:

d = S + S (3) o i (3)

Rearranging equation 3 and substituting Si into equation 2 and solving for So we obtain:

d ± d 2 − 4df S = (4) o 2

This provides for two real image solutions provided that f ≤ d/4. We can then represent lens position 1 and position 2 from the object respectively as:

d − d 2 − 4df = (5) S1o 2 (5)

d + d 2 − 4df S2 = (6) o 2

To determine the magnification relation between image 1, determined from lens position 1, and image 2, determined from lens position 2, we use the following relations:

S1 m1 = − i (7) (7 ) S1o

S2i m2 = − (8) S2o

S1 S2 m1⋅ m2 = i ⋅ i (9) S1o S2o

Where m1 is the magnification of image 1, and m2 is the magnification of image 2. The negative sign in front of each equation indicates that the image is inverted. Solving equation 1 for S1 i and S2 i, in a similar manner as was done in equation 2, gives:

32

S1 f S1 = o (10) i − S1o f (10)

S2 f S2 = o (11) i − S2o f

Substituting equations 10, 11 and then 5, and 6 into equation 9 gives the relation between magnifications as:

1 m1 = (12) m2

thus showing that the magnifications of image 1 and image 2 are the inverse of one another.

If the focal length of the lens is unknown, it can be found when the magnification of one of the images is known and the distance from the object to the lens is known (or distance from the lens to the image is known). In addition to equation 7 and 8, the magnification of the image can be defined as the following:

yi Si m = − = − (13) (13) yo So

where yi is the size of the image and y o is the size of the object and both sizes are positive values and the negative sign indicates an inverted image. This is equivalent to the magnification as defined in equation 7 and 8. Once the magnification is found by measuring yi and yo, and knowing that: = − ⋅ Si m So (14) (14) then substituting equation 14 into equation 2 allows one to solve for the focal length f below:

S ⋅ m f = o (15) m −1 (15) keeping in mind that m is negative as defined above when working with real images and a single lens.

33 Specific Example Consider a lens that has a focal length of 100mm and we make the object to image distance of 450mm. Substituting the above values into equation 5 and 6 to find the object to lens distance at position 1 and position 2 provides:

450 mm − (450 mm )2 − 4 ⋅ 450 mm ⋅100 mm S1 = = 150 mm o 2

450 mm + (450 mm )2 − 4 ⋅ 450 mm ⋅100 mm S 2 = = 300 mm o 2

Utilizing equation 3 to find S1i and S2i gives:

= − = = − = S1i 450 mm 150 mm 300 mm S2i 450 mm 300 mm 150 mm

Substituting the above information into equations 7 and 8 yields:

− 300 mm −150 mm 1 m1 = = −2 m2 = = − 150 mm 300 mm 2

Both images are inverted with image 1 being 4 times the size of image 2. Figure 7 illustrates this example.

34 Lens Position 1

Object

Image 1

(a)

150mm 300mm

Lens Position 2

Object Image 2

(b)

300mm 150mm Figure 7: (a) Projection lens with a 100mm focal length at position 1 where the distance from the object to the thin lens at position 1 is 150mm and the distance from the lens at position 1 to the image is 300mm with image 1 having a magnification of –2. (b) Using the same projection lens in (a) at position 2 where the distance from the object to the thin lens at position 1 is 300mm and the distance from the lens at position 1 to the image is 150mm with the image 2 having a magnification of –1/2.

Useful information For more information on imaging please see the listed references below.

- Frank L. Pedrotti, S.J. and Leno S. Pedrotti, “Introduction to Optics, Second Edition,” (Prentice-Hall, Inc. 1993) - http://www.hazelwood.k12.mo.us/~grichert/optics/intro.html (Optics Bench - A lot of fun!)

35

Optical system demonstration Anatomy of a camera

Equipment Kodak FunSaver35 Camera One flathead screwdriver from eyeglass repair kit (or other similar tool to pry the camera open) Two pen caps

Set up and disassembly 1. Remove camera from packaging. Slowly and gently peal off front and back label with fingers. See Figure 1 to see camera with label.

Button to charge flash

Figure 1: Camera with label

Caution : Before step two, please note that there is a AA size battery that charges the capacitor which then powers the flash. If you do not know if the flash is charged (if the button to charge the flash was inadvertently pushed), then take a picture with the camera and let sit for 30 minutes to avoid risk of an electrical shock.

2. Pry open the camera using a small flathead screwdriver and pen caps. This is done by wedging the screw driver and pen caps under the top and bottom of the camera and prying open the left and right sides of the camera as indicated in a series of pictures in Figure 2 on the top, bottom, left and right sides. To facilitate opening the camera, once the screwdriver has pried open the top of the camera, slide in a pen cap before removing the screwdriver and then remove the screwdriver when pen cap is in place. Then repeat this last step for the bottom as shown in Figure 2a and b. Now pry open in left side (camera facing you) with the screwdriver only as shown in Figure 2c and then pry open the right side with the screwdriver only as shown in Figure 2d.

36 Wedge screwdriver Wedg e screwdriver and first pen cap and second pen cap here here

(a) (b)

Pry open left side with screw driver Pry open right side with screw driver

(c) (d) Figure 2: (a) Top view (b) bottom view (c) left view (d) right view

3. Once the camera is open, remove the AA size battery and film. Figure 3 shows the camera opened, but with the film and battery still in place. Figure 4 shows the camera with battery and film removed. Front and back case

Battery Film

Figure 3: Camera is first opened and lying on its face

37

Figure 4: Camera with battery and film removed

4. Flip camera such that it is facing you and identify the optical components shown in Figure 5. Gently push down and turn counter-clockwise the gray retaining ring holding the camera lens and remove the retaining ring and lens. Remove the lens for the flash by pulling and wiggling it out. Next, tilt the camera such that you are looking at the top of the camera and remove the view finder by gently pulling it up and away from the camera. A view of the components is shown in Figure 6.

View Finder Lens for Flash Camera Lens

Retaining Ring

Figure 5: Exposed camera highlighting optical components

38

Figure 6: Exposed camera with optical components removed

Exploration 1. Explore the optics. Find bright light sources and image them on to paper, tables, walls, etc. Roughly determine the focal length of each optic and see how they differ. The camera lens provides a sharp image on the film plane. The lens covering the flash is very interesting. It has a simple cylindrical lens on the outside and a cylindrical lens in on the inside in the other dimension. The view finder has four optical components. The positive lens is where your eye looks through and the negative lens faces out to your scene that you are taking a picture of. Together, they demagnify the view. The Light Pipe guides light from the red LED, when the flash is charging, to the outside edge of the camera. See Figure 7

Short focal length lens provides Positive lens magnification for indicator for number of view finder of pictures left Negative lens of view finder

Light Pipe for LED to indicate when the flash is fully charged

Figure 7: Optics components of camera

39 2. Place all of the optics back on the camera as they were previously in Figure 5. Place a piece of Scotch tape along the center back of the camera where the film would be. Now point the camera at a distant indoor light source or out a window to see the inverted image on the tape. See Figure 8.

Figure 8: Scotch tape in film plane

3. Explore the mechanical function of the camera. Place the camera down on its face. Turn the gray wheel with spokes where the film would be and this prepares the camera to take a picture. See figure 9. Press the gray button on the top of the camera to snap the shutter open briefly.

Turn this wheel here

Figure 9

For those who wish to further investigate the mechanics and electronics, the camera will further disassemble allowing for a deeper understanding of those functions.

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Follow up discussion/questions - Is the image inverted on the film plane? Why?

- Since the shutter speed is always the same for this camera, what does that tell you about using the camera outdoors on a sunny day versus indoors using the flash?

- Why does the view finder demagnify the view?

How it works There are four essential elements in a single-use-camera. The lens, the film, shutter, and camera body as shown in Figure 10. The lens is already set at the correct distance to form an image on the film. The film is chemically altered by the light and records the image. The camera body keeps the inside of the camera dark so as to not have any extraneous light expose the film. The length of time that the shutter is open, typically a fraction of a second with single-use-cameras, then controls the amount of light that can expose the film. The old advertising slogan of Kodak still holds for this camera. “You press the button, we do the rest”.

Camera body

Shutter

Film Lens Inverted image of distant scene

Figure 10: Essential elements of a single use camera

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Useful information For more information on how cameras work, see the references below.

- David Falk, Dieter Brill, and David Stork, “Seeing The Light: Optics in Nature, Photography, Color, Vision, and Holography ,” (John Wiley & Sons, Inc. 1986) - http://entertainment.howstuffworks.com/camera.htm/printable - http://electronics.howstuffworks.com/digital-camera.htm/printable

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