Abstract

If you sit under a leafy tree on a sunny day, you may notice spots of sunlight on the ground from light passing through spaces between the leaves. Try putting a piece of cardboard on the ground and examining the spots of light on the cardboard. Even though the spaces through which the light is passing are irregular in shape, the spots on the cardboard are round. What you are seeing, in fact, are projected images of the sun. Light passing through an aperture forms an image. A pinhole camera uses a tiny aperture, instead of a fancy lens, to project an image. What happens to the quality of the picture as the size of the pinhole is changed? This project shows you how to find out.

Objective

The goal of this project is to measure the resolution of a pinhole camera as a function of pinhole diameter.

Introduction

Light passing through an aperture forms an image. Sunlight passing through spaces between the leaves on a tree projects an image of the sun. Make a loose fist with your hand on a sunny day and you can project an image of the sun through the aperture that your fingers make. For a great introduction to light and apertures, including many interesting demonstrations you can do yourself, see Bob Miller's "Light Walk" pages, in the Bibliography section (Miller, date unknown).

The resolution of the image projected from an aperture is determined by the size of the aperture (until the aperture becomes so small that diffraction of light dominates). Thus, the smaller the aperture, the greater the resolution of the image. When the size of the aperture approaches the wavelength of the light passing through it (roughly 400–750 nm for visible light) diffraction of the light by the aperture limits the resolution.

In this project, you will build a digital pinhole camera. You'll make several different-sized pinhole apertures for your camera, and you'll measure the camera's resolution as a function of aperture size. You can build a pinhole camera for film, but loading and unloading the film has to be done in total darkness. Also, you have to have the film processed before you can see the results. Determining the correct exposure time for a pinhole camera often involves some trial and error, and this can quickly become tedious when using film. A big advantage of using a digital camera is that you can see your results right away.

Since a pinhole does not let in much light, exposure times will be much longer for a pinhole camera than for a camera with a normal lens. Therefore, you must use a digital camera that provides a method for controlling long exposures (e.g., via a remote-control cable). For the lightbox camera described, exposure times may need to be tens of seconds, or even a minute.

There are two ways to make a pinhole camera with a digital camera. Which method you use depends on the type of digital camera you have. If your digital camera does have removeable lenses, you can make a pinhole for it using a spare body cap with a pinhole in place of the lens. With this type of camera, the light coming through the pinhole aperture forms an image directly on the light-sensitive component inside the camera.

If your digital camera does not have removeable lenses, you will use the "lightbox" (or camera obscura) method. With the lightbox, the pinhole projects an image onto a screen at the back of the box. Your camera takes a picture of this screen, through a small, light-tight "window" at the front of the box (see Figure 1).

With this type of camera, the exposures are even longer than typical pinhole cameras, because the light from the pinhole does not fall directly on the light-sensitive region of the camera. Instead, the camera is capturing some of the light that is reflected from the projection screen. This, in turn, is only a fraction of the light that originally passed through the pinhole.

The resolution of the image will be determined by how much the light is "spread out" as it is projected through the pinhole. With larger pinholes, this will be determined by the diameter of the aperture. As the pinholes become smaller, diffraction will begin to have a significant effect. Diffraction ultimately limits the resolution of an optical system, even something as simple as a pinhole.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• Aperture
• Diffraction

Questions

• How small does the pinhole have to be before diffraction becomes dominant in determining resolution?

Materials and Equipment

• Flat black paint
• Scissors or a hobby knife
• Soda can
• Tin snips
• Needle
• Scrap wood (a few pieces)
• Sandpaper, coarse and fine
• Digital camera. It must be able to take long, controlled exposures without flash.
  • If you are using a digital camera that has removeable lenses (i.e., a digital single lens reflex [DSLR, or SLR] camera), you will also need:
    • Spare body caps (at least 3). You should be able to order these from the camera store where you purchased the camera. You will make a pinhole for the camera using a spare body cap with a pinhole in place of the lens.
    • Drill
    • Glue
    • Black tape
  • If you are using a digital camera that does not have removeable lenses, you will use the "lightbox" (or camera obscura) method. To make a lightbox, you will also need:
    • A box
    • Aluminum tape
    • Rubber from bicycle inner tube
• Optional: Photo editing software will be useful for analyzing results.
• Lab notebook

Experimental Procedure

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Photography, Digital Photography & Video Project Ideas
• My Favorites

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