Cameras have optical systems, or lenses, made up of several separate pieces of glass, called elements. There are two reasons for having several elements. First, it allows the designer to make many different types of lenses to suit different purposes. Second, the quality of the image formed by the lens can be controlled by choosing different lens elements. The most important choices the lens designer makes are the shape and position of each lens element. These govern properties like focal length, angle of view, physical weight, and size.

Lenses are probably the least understood but the most discussed component of the photographic process. Photographers (generally amateurs) speak of a lens formula as if they knew what it was about. Even if the designer's formula were made available, it would not provide information about the lens photographic quality. A perfect lens cannot be made. A lens is a compromise of inherent errors called aberrations, but do not let this worry you. Lens aberrations are defects in the formation of an optical image. Today's lenses are so highly corrected for lens aberrations that, except for a few ultra wide-angle (fisheye) lenses, you would be hard pressed to find a lens that produces subjectively identifiable

Figure 1-15. Plane reflector and subject coverage.

aberrations. You may hear photographers talking about aberrations as if they were important. They may make an interesting subject, but knowing all the details about them does not help you to take better photographs. Important matters that will improve your skill as a photographer are knowing how to control the factors, such as exposure, composition, lighting, and lab work. Let the lens designers and manufacturers worry about the lens aberrations. However, just so you know what these lens aberrations are, a brief definition is provided for each of them in the glossary; they are as follows:

- Astigmatism

- Chromatic aberration

- Coma

- Curvilinear distortion

- Spherical aberration

Today's lenses can image more detail than present film materials can record. Therefore, avoid discussing lens resolution. If you want to discuss resolution, talk film resolution.