The Basics
A camera lens builds upon the principles of a basic lens, which bends light to produce an image on a projection surface. The human eye is an example of this system, which allows light to enter through a curved surface to reproduce an image behind that surface. This bending of light is known as refraction.
Components
The camera lens is so-named because, originally, a camera only used one lens to focus and create an image. One concave or convex surface was moved closer or further from the projection surface to adjust the desired picture. As more features have been added to cameras, more lenses have been added to a camera's lens housing to help control zooming, focusing and other actions. Modern cameras use what is called an "objective" rather than a lens. The difference is that an objective uses more than one internal lens. Each lens is called an element, and the number and quality of elements play a direct role in the cost of a camera lens.
Lenses and Light
Regardless of the number of elements a camera lens has, one basic principle remains. Because lenses are used to re-create pictures, light is an essential component in photography. As with the human eye, too much light entering through the lens will make an image that is flooded with light. Too little light creates a darker image. The human eye adjusts automatically (test this by walking from a well-lit area to a very dark area, and see how long it takes for your eyes to adjust to the light). However, most cameras allow photographers to correct a lens' aperture, giving the user the ability to control the light entering a camera lens.
Elements
Early zoom cameras were based on the designs of compound telescopes, which allowed a user to zoom in and out, and focus the field of view. Today's high-end camera lenses may use as many as 30 different elements, though a much lower number generally suffices for a typical camera zoom lens. The element housing is designed to keep specific elements fixed in place while turning and moving others at various speeds to create sharp images at any setting that a photographer might want.
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