To create any kind of image you need to focus the rays of light from the subject on to the film or sensor. Normally, this is done with a lens, but a very small hole will do a similar job. The quality is not that good, but the effect is very interesting.
Firstly, there's no need to focus. A lens produces rays of light which converge to a point at a specific distance behind the lens, whereas the hole in a pinhole camera produces rays of light without any convergence.
As a result of this, the depth of field is essentially infinite. Subjects at any distance from the camera will come out with more or less equal sharpness (not that pinhole images are all that sharp anyway, mind).
You can make your own pinhole camera with a digital SLR and a spare body cap (see the illustration above). You need to drill a hole in the body cap, then tape some tinfoil over this hold and use a fine needle to make a 'pinhole' in the foil.
Then you'll need to set the camera to manual exposure mode and experiment with the shutter speeds to find the best exposure. You could try using a handheld meter (illustrated), but unless you know the approximate aperture value of your pinhole, you're always going to be guessing.
Even pinhole cameras have focal length and aperture values. The focal length is the distance between the pinhole and the sensor, while the aperture value is this distance divided by the size of the hole.
Getting the size of the hole right is a bit of an art. If it's too large, the light rays will diverge too much and the photo will be blurred. If it's too small, the image will be blurred again, this time because of diffraction effects. Finding the 'sweet spot' can take a few attempts.
This is an image shot with the setup in the illustration above. It's pretty soft, and this is because of the size of the sensor. Pinhole photography worked well with big, sheet film cameras, but the APS-C sensor in a digital SLR has a fraction of the area.
Despite this, though, pinhole photography has a certain fascination, even when carried out with a modern digital SLR. It's quite uncanny being able to capture a recognisable image without any kind of lens at all, and a reminder that the optical principles behind photography remain at least as fascinating as the latest digital technologies.
