Penland Science in the Studios: The Pinhole Camera

Three Jeffs, image made with a three-pinhole camera
Science in the Studios class, Fall 1999

Introductory Science Concepts: Hands-On Activities:


Students are introduced to photography through a discussion of the fundamental nature of light waves and the interaction of light with matter. Then they learn about the chemical changes that take place when photographic film or paper is exposed to light, developed, and printed. Students use homemade pinhole cameras to obtain images. They develop and print their pictures in a darkroom.

Introductory Science Concepts

Light is electromagnetic energy that travels through space in straight lines. When light encounters matter, it may be reflected, refracted, or absorbed. If the surface of the  object is not perfectly smooth, the reflected rays are bounced off the object at many different angles. Light rays reflected from an object may be thought of as originating at the object.

[Physical Science 4.02]
[Physics 7.03]

The Pinhole Camera
How does the pinhole image work?

Imagine a person standing in front of wall.  Ordinarily, light rays bouncing off the subject will not form an image, because rays traveling in a straight line from each point will strike the wall in many different places.  The wall will be hit by reflected light, but there will not be a focused image.

If we place a pinhole between the subject and the wall, and if we allow no other light to hit the wall, the light reflected off each point on the subject will only hit one point on the wall beyond, and an image will be formed.  This principle was understood long before the invention of photography, and was know as the camera obscura, or darkened room.

Why can't we make the pinhole bigger to allow in more light?
The bigger we make the pinhole, the less sharp the image becomes, because rays from each point on the subject hit more than one place on the wall or film.

Why is the image upside down?
Since light travels in a straight line, rays from above the pinhole pass through it and hit the bottom of the wall opposite, while rays from below the pinhole end up at the top of the back wall.
How can the pinhole image be captured?
By placing a piece of light sensitive material (photographic paper or photographic film) on the wall, opening the pinhole for a long enough period of time to expose this material, and then developing this material, we can make a permanent record of the pinhole image.
Why doesn't the image have much color?
The retinas in our eyes have two types of receptors: rods and cones.  Rods are much more sensitive, but they only register light and dark.  Cones allow us to see color, but they need more light in order to work.  Since the pinhole only allows in a small amount of light, we see the image primarily with our rods, and thus largely in black and white.

[Biology 3.03]

The Chemistry of Photography

A camera uses light sensitive photographic film or photographic paper to record and preserve the image produced by light rays entering through the pinhole. Photographic film and photographic paper consists of a support (plastic film or paper) which contains a thin film of resin or gelatin. Embedded in the gelatin are very small crystals of silver chloride, silver bromide, or a mixture of these two silver halide salts. If light rays strike these crystals, the light is absorbed and the energy of the light is used to produce a small speck of silver metal as described by the following chemical equation (using silver bromide as an example).
light energy 
silver chloride 
metallic silver
bromine atom

[Chemistry 2.03]

Both the silver and the chlorine atoms produced in the reaction remained trapped in the gelatin. Only a tiny fraction of the silver in the silver halide crystal is affected by the light. Crystals of silver halide that have not been exposed to light undergo no chemical change. The light-altered silver halide crystals on the film or paper are called the "latent image".

The process described above is made more understandable to the students in the following way. The students gather as a group. Each student holds two large circles of white paper (paper plates) in front of him or her. One circle is labeled Ag+ and the other is labeled Br -. The Br - circles have two small pieces of paper, each representing an electron, taped to the bottom. Thus, each student represents a silver halide ion pair. The instructor or another student, pretending to be a light ray, runs over to the group, transfers an electron from the Br - plate to the Ag+ plate and reverses the Ag+ plate to reveal a black side labeled Ag. The "light ray" performs this transfer on only a few of the "ion pairs" in the silver bromide crystal. The students, whose paper circles have been reversed to black, represent the silver atoms that are formed in the latent image. The light ray served as an energy source that allowed the positively charged silver ion to "take back" an electron from a bromide ion, thereby becoming a silver atom. The gain of an electron by the silver ion is an example of a chemical process known as reduction. The loss of an electron from the bromide ion is oxidation.

Film or paper containing a latent image is developed by an alkaline aqueous solution of a reducing agent. This process must be done in a dark room. The atoms of metallic silver in the latent image act as a catalyst to accelerate the reduction of the remaining silver ions in each of the "exposed"  silver halide crystals. Hydroquinone is commonly used as the reducing agent. The following chemical equation describes the process.
 2  AgBr
2  NaOH
2 Ag 
 2  NaBr
 2  H2O
silver bromide
sodium hydroxide
metallic silver

[Physical Science 6.01]
[Chemistry 2.05]

Returning to the human drama, the instructor or some other students may pretend to be the hydroquinone molecules. Each hydroquinone molecule gives up two electrons to two silver ions. The white paper circles held by two students acting as silver bromide ion pairs may be turned to black by each person acting as a hydroquinone molecule. Since the hydroquinone molecule loses two electrons in the process (is oxidized), it must convert to a benzoquinone molecule. The hydroquinone actors may carry black circles that change to white after they have reacted with two silver ions.

The black colored metallic silver produced by the reaction remains trapped in the gelatin or resin. Silver bromide crystals that were not exposed to light do not react with the hydroquinone during the short amount of time (one minute or so) that the paper or film is immersed in the developing bath. They remain in the gelatin because they are not soluble in water.

After the prescribed amount of time, the development process is ceased by removing the paper or film from the developing bath and placing it in a solution of acetic acid (HC2H3O2). This solution is called the "stop bath". The acetic acid reacts with alkaline NaOH in the developing solution as shown by the following equation. In the absence of NaOH, the reduction of silver ions by hydroquinone ceases.
sodium hydroxide
acetic acid 
sodium acetate

[Chemistry 4.04]

 To illustrate this process in the drama or skit, any extra hydroquinone actors may leave the scene.

 The paper or film is then placed in a fixer bath, a solution containing a substance that removes the unexposed silver halide crystals from the gelatin. The most commonly used reagent for this purpose is sodium thiosulfate (Na2S2O3). It reacts with silver ions to form water soluble, complex silver salts as in the following equation.
AgBr + 2 Na2S2O3 -----> Na3[Ag(S2O3)2] +  NaBr
silver bromide sodium thiosulfate soluble complex silver salt

[Physical Science 6.04]
[Chemistry 2.04]

The metallic silver produced in the developing process does not react with sodium thiosulfate and so remains on the paper or film.
Students acting as "fixers" may come in and take away any students in the original group of silver bromide ion pairs whose circles are still white. These are the silver and bromide ions that did not react with light and did not react with the hydroquinone. The students remaining represent the image on the film or photographic paper.

The paper or film is then placed in water to remove all the soluble silver salts produced in the fixer bath. After rinsing the film or paper contains a "negative" image of the object. Whatever was white or light colored on the object appears as a dark spot of silver in the negative.

A positive image of the object may be produced (from exposed photographic paper) by placing the negative imaged paper face to face (gelatin sides together) with another, unexposed piece of photographic paper. This sandwich is exposed to light (negative image on top) for a specified period of time. The exposed fresh paper is then processed as before.

Hands-On Activities

The Camera Obscura
Students can observe the principles of image production by a pinhole during this session. There is a Camera Obscura, large enough to accommodate several people, that has been erected in a meadow on the campus of Penland School. The Camera Obscura is a dome structure that is entered by a spiral path leading to a very dark inner chamber. The inner chamber is illuminated only by light entering through a small hole in the outside wall. When the students in the chamber become accustomed to the low light, they are able to discern the upside down image of the outside landscape on the wall opposite the small hole. Since the Camera Obscura can only accommodate five or six people inside, the remaining students position themselves outside the camera so that their images are also projected on the wall.

The experience of the Camera Obscura makes the abstract discussion of light waves and image production into a sensory experience that students absorb readily and thoroughly.

Making A Pinhole Camera
The set of instructions below is given to students so that they can make their own pinhole camera, if desired.  For the session, several cameras are already constructed through step 3. Students finish the construction and load the cameras in the darkroom with photographic paper. Students are encouraged to obtain one exposure, outdoors, with the camera supported on the ground or a stable structure, using an exposure of 7-15 seconds depending on the sun's brightness. Once they have obtained this first image, and have successfully developed and printed it, they are eager to try other techniques. The instructors are there to answer questions, help correct mistakes, and suggest ways of producing desired effects. Students have time to obtain, develop, and print several images.

Mr. Jeff Goodman of Appalachian State University
helps a student make a coffee can pinhole camera

How to Make Your Own Pinhole Camera

1. Get a light-tight container that you can open and close.  If you use a coffee can, make sure you put some light tight plastic, such as a film bag, under the lid.

2. If the inside of the container is reflective, paint it black (spray paint works best).

3. Drill or punch a big hole (approximately 1/2" diameter) through one side of the container.  Take off sharp edges with a file if needed.

4. Tape a square of pie plate aliminum over the hole on the inside of the container.  Make sure the tape is only on the edges of the metal and that all edges are taped securely (so no light can leak around them).

5. With a pin, drill a hole through the foil where it shows through the larger hole.

6. Place a piece of electrical tape over the pinhole on the outside.  This is your shutter.

7. Load your camera in the darkroom with any light sensitive material (paper or film); make sure the film or paper doesn't block the pinhole, and that the emulsion side is toward the hole.  Secure the top so that light won't leak in.

8. Take your camera out and set it up opposite your subject.  Make sure the camera and the subject can stay absolutely still for the exposure (7 seconds in bright sun if using a small coffee can with 5x7 Ilford Multigrade IV RC).

9. Take the tape shutter off to expose your image.  Replace it when the exposure is done.

10. Go back in the darkroom and develop your film or paper normally.

Positives can be made from Ilford RC paper negatives by wet-sticking the paper negative to unexposed paper with the emulsion sides touching, exposing the sandwich (negative on top) for 20 seconds at f-4 with a #4 filter.


Dr. Claire OIander, Appalachian State University
Mr. Jeff Goodman, Appalachian State University

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