The simplest camera is a pin-hole camera, just a box with a pin-hole in one end of it to allow some light to come in as sketched here. At the end opposite this pin-hole we can place a piece of film to record the image or a ground glass to view the image from behind the camera. (You can make such a pin-hole camera yourself; a piece of waxed paper makes a very adequate substitute for a ground glass).
From each point on the object there are rays of light leaving in every possible direction. A few of the rays of light coming from the top of a tree are shown in the sketch. Only one of those rays passes through the pin-hole and arrives at the back of the camera. That is true for every point on the object. From each point on the object only one ray of light will pass through the pin-hole. Each of these rays that pass through the pin-hole will have a different direction, as illustrated below for rays coming from several different points on the tree. That means each of these rays strikes the back of the camera at an appropriate place to produce a clear, sharp image.
With such a tiny opening or aperture to admit light in a pin-hole camera, the image may be dim. Using a larger aperture (perhaps a "nail-hole" instead of a pin-hole) will make the image brighter. But, with more rays of light from each point on the object, the image becomes blurred. As indicated below, light from each individual point on the object now creates a blurred circle on the image and these fuzzy dots overlap. The image is now brighter&endash;but blurred.
A lens in the opening can be used to focus the image as shown below. Now the image will be sharp only for a particular object distance do. Usually the focal length of the lens f is fixed. To focus on nearby or far away objects requires that the image distance di be changed; when you focus a camera, you are adjusting this distance between the lens and the film.
(c) Doug Davis, 2002; all rights reserved
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