THE CAMERA

There are a number of similarities between the human eye and a simple camera. Both have an iris to control the amount of light entering, and both record an image at the focal plane of the lens. In a camera, the focus is adjusted, not by changing the shape of the lens as in the eye, but by moving the lens back and forth. The eye is somewhat like a TV camera in that both record images at a rate of about 30 per second, and the information is transmitted electronically to either the brain or a TV screen.
On many cameras you will find a series of numbers labeled by the letter f, called the f number or f stop. Just as for the parabolic reflectors in figure (1), the f number is the ratio of the lens focal length to the lens diameter. As you close down the iris of the camera to reduce the amount of light entering, you reduce the effective diameter of the lens and therefore increase the f number.

Figure 1: The lens system for a Nikon single lens reflex camera. When you take the picture, the hinged mirror flips out of the way and the light reaches the film. Before that, the light is reflected through the prism to the eyepiece.