The purpose of a camera lens is to control the light rays entering the camera. The simplest kind of lens is a pinhole in a piece of thin metal or black paper. Of course, only an extremely small part of the light reflected by a subject passes through the pinhole and enters the camera. When the pinhole is large, it allows more light rays to enter but blurs the image. This blur is really an overlapping of several images. Images produced by large and small pinholes are the same size, but one is blurred, while the other is sharp. A photographic lens is a piece of polished and carefully shaped glass that refracts light rays so an image of a desired scene is formed on the rear wall of a camera. A lens transmits more light than a pinhole. It increases the brightness and improves the sharpness of an image. The basic principle of a lens-any lens-is relatively simple.

First, consider an image formed with a single pinhole. Next, consider another pinhole above the first. This pinhole forms a second image. When these two images could be made to coincide, the result would be an image twice as bright as the original. Now, consider a third pinhole on the side of the first, a fourth on the other side, and a fifth below the first. All four pinholes project separate images slightly removed from the first or center one. When these four images are made to coincide with the center one, the result is an image five times as bright as the image made by the one center pinhole. By using the principle of refraction, you can make these four images coincide with the center one. By placing a prism behind each pinhole, you are causing the light that forms each of the four images to be refracted and form a single image. In other words, the more pinholes and prisms used, the brighter or more intense the image. A lens represents a series of prisms

Figure 1-16. Formation of an image by a lens.

incorporated in a single circular piece of glass (fig. 1-16).