Personal Pinhole Theater

Before you read on, take a good look at the figure below to get an idea of what you’ll be doing with the box.

1. Using transparent tape, attach the sheets of white paper to the inside front wall of the box (the side you’ll be facing when your head is in the box). This is your viewing screen. With a pencil or marker, mark this side of the box on the outside.
2. Seal the whole box shut with duct tape.
3. Use the utility knife to cut a hole in the bottom of the box through which you can just barely fit your head (see photo below). Position the hole so that the back of your head will rest against the wall opposite the screen.
   
4. Hold up the box up to the light and look in through the newly cut head hole (see photo below). Other than the head hole itself, all corners and seams should be sealed against light leakage. If there are light leaks, cover them with duct tape on the outside of the box. You may need multiple layers of tape.
   
5. Cut a small hole, 1.5 inches (4 cm) square, in the side of the box opposite the screen. The hole should be at least 4 inches (10 cm) above where the back of your head will be located when it’s inside the box. (See the diagram above to ensure proper location of this hole.)
6. Cut a flat square of aluminum foil large enough to cover the square hole you made in Step 5.
7. Place the square of foil over the square hole and, using masking tape, tape along all four sides to prevent light leakage (see left photo below). Tape along the edges only; don’t place any tape over the center of the foil square (see right photo below).
   
8. Use the pushpin to poke a single pinhole in the center of the aluminum foil, and that’s it: You’ve built your own Personal Pinhole Theater.

On a bright, sunny day, take your Personal Pinhole Theater and dark fabric outside. Put your head into the box, positioned so you’re facing the screen. Wrap the fabric around your neck to keep light from leaking into the box from the bottom (see left photo below). The photo on the right below shows a "cutaway" of the interior of the box.

As your eyes adjust to the dim surroundings (this might take a few minutes), notice if anything appears on the screen. When images do appear, notice their orientation. Are they right side up, or upside down? Are they left-right reversed, or normal?

Carefully move around and notice how the image on the screen changes. Try to position yourself so that two similar objects at different distances away (such as two cars) cast their images on the screen at the same time. Which image is larger? Are both images sharp and in focus at the same time?

After you’ve had a chance to use your Personal Pinhole Theater for a while, try enlarging the pinhole by carefully poking a pencil point through until it’s about half to two-thirds the diameter of the pencil (see photo below). What happens to the image on the screen? If you want to go back to the small hole, just remove the square of foil and tape on a new one. Poke another pinhole and you’re ready to go again.

Your Personal Pinhole Theater is actually a giant camera obscura (Italian for “dark room”), or pinhole camera. Light rays from the sun reflect off every point on every object—including, say, a tree. The rays from the tree then hit the outside of the box, except where they can pass through the pinhole. Each light ray, in effect, carries an image of the point on the tree where it originated.

The pinhole lets only a limited number of rays from each point pass through. The rays from each point are projected on a small area of the white viewing screen, without significant overlap from the images of the other points. The result is a clear image of the tree. Any images on the screen are upside down and left-right reversed. (The diagram below shows how the up-down part of this reversal takes place; light rays from only three points are shown for clarity.)

As you can see, the ray from the top of the tree passes through the pinhole and hits the lower portion of the screen. The ray from the bottom of the tree also passes through the pinhole, but hits the upper portion of the screen. This geometry results in the image of the tree appearing upside down on the screen (and left-right reversed, as well).

The farther an object is from the pinhole, the smaller its image will be on the screen (see the diagram below). The focus, or sharpness of the image, is not affected by distance.

Using a larger pinhole gives you a brighter image, since it lets in more light, but increases the overlapping of images. The result is an image that’s lost its sharpness, becoming blurry. The photos below show the image inside a Personal Pinhole Theater pointed at the San Francisco skyline with increasing pinhole sizes.

There are many ways to explore and experiment using your Personal Pinhole Theater. Here are just a few:

Pinhole Variations
Systematically vary the size or shape of the pinhole and note how the image changes. You can also try using two pinholes and see what happens.

Walk right in!
There are giant, walk-in camera obscuras all around the world. See if there’s one nearby that you can visit. Or create one of your own by making a Personal Pinhole Theater from a refrigerator box that’s big enough for your whole body.

Artists’ Tool
Some people think that Renaissance artists used portable camera obscuras very much like your Personal Pinhole Theater to create their masterpieces. This hotly contested theory can make a great research project.

Photography
If you teach about light or photography, the Personal Pinhole Theater demonstrates a classic tradeoff in optics: balancing brightness with sharpness.

In photography, the zone in which objects will be in sharp focus is known as depth of field. The smaller the lens aperture or camera lens opening, the greater the depth of field. In other words, the more the lens opening is like a pinhole, the greater its ability to have objects at different distances simultaneously appear in focus. As the opening is made smaller, however, less light is allowed through, and you have to use a longer exposure time to compensate. To take a picture in dim light, you open the lens wider to let in more light, but in so doing you sacrifice depth of field.

Your eyes
This Snack can also be used to show interesting physiological properties of the eye. Your eyes can function over nine orders of magnitude of brightness. However, you might notice that it takes time for your eyes to adjust from bright surroundings to the darkness inside the box. This is called dark adaptation. It might take up to 30 minutes for your eyes to fully adapt to near darkness, allowing your eyes to be up to a million times more sensitive to light than at full daylight.

The light sensors located on the retina at the back of your eye are called rods and cones. Rods only detect light and dark (or black and white). They’re highly sensitive and can be activated by only a few photons striking them. Cones, on the other hand, sense different colors of light, and need much more light to be activated. That’s why the pinhole image projected inside your Personal Pinhole Theater is usually only seen in black and white. If you enlarge the hole, allowing more light to flood in, you can see the image in color.

It may be helpful to do this Snack using partners. Since someone using a Personal Pinhole Theater needs time to become “dark adapted,” having a partner poke the pinhole in the aluminum foil, and subsequent larger holes, means that the user can stay inside the box to watch any changes take place.

A partner can also slowly and carefully guide the user to an unknown location before poking a pinhole into the foil. Users can try to guess where their partner has taken them or just be surprised by the pinhole’s projected image. This is a great icebreaker or trust activity.

Watch this video to see Teacher Institute staff present this activity in a workshop designed to help teachers bring Science Snacks into the classroom.