Thread: The Visual System - Free Course from MIT

How we see is best understood by comparison with a camera. The camera uses a shutter with a small, central opening to admit light. If the object is bright, the shutter closes the opening; if dim, the shutter enlarges the opening to collect more light. The camera lens focuses the light, bringing it to a sharp image, on photographic film. Because this film is at a fixed distance behind the lens and because viewed objects lie at various distances, different-shaped lenses are needed to bend light rays to a focus. For close objects, a more "bulging," or convex ("zoom"), lens is needed for greater bending. A thinner lens, producing less bending, is needed for more distant objects. Film images "develop" because the focused light patterns react chemically with light-sensitive "receptors" on the film. A camera case encloses and protects the shutter, lens, and film.