The lens is the image-forming device on a camera. It my be composed of from one to as many as 10 or 12 elements. The first cameras had a single-element meniscus lens (a lens with one concave and one convex surface). Besides the very low speed, this type of lens suffered from a number of optical defects and was replaced with greatly improved, more complex lenses.

There are three basic types of lenses: normal, wide angle, and telephoto. The lens's focal length--the point at which light rays focus through the lens--determines the size of the image that will be produced on the film.

With a normal lens, the viewing fieled is approximately 50 degrees. The objects appear normal in size and shape, relative to the picture's background. A camera that uses a 35-mm film will usually have a 50-mm lens for normal coverage.

In a wide-angle lens, the field of view is much wider: about 90 degrees. These lenses are used where the distance between camera and subject is limited, as in interior photography. The wide-angle lens is also used to make smaller objects look larger (to give a spacious impression of a small room, for example), or to photograph large objects form close up.

Telephoto, or long-focus lenses, have a smaller field of view than a normal lens, and show an enlarged detail of the image over the same film area. Interchangeable-lens cameras offer the photographer the opportunity to select a focal length that is best for any given situation. In recent years, variable-focal-length, or "zoom," lenses, have become very popular. A single lens of this type can replace many individual lenses, and offers a great convenience to the photographer.

The speed, or light-gathering power, of a lens is indicated by the f number, called the aperture. The lower the f number, the faster the lens--that is, the more light it lets through. A fast lens has an aperture of at least f/2.0.

One of the two factors that determines the correct film exposure in the amount of light allowed to pass through the lens. Mechanically reducing the aperture improves optical performance, particularly toward the edge of the picture. It also increases the depth of field, which is the zone of good focus. Most cameras use an iris-type diaphragm, which consists of a number of very thin metal blades. They are mounted so that by rotating a ring or moving a lever, the size of the lens opening can be varied. On automatic cameras the diaphragm is adjusted by a built-in-mechanism to produce the optimum exposure over a wide range of lighting conditions.

The various openings of the diaphragm--called f-stops-are stamped on the lens mounting. Each change of diaphragm opening changes the amount of light passing through the lens by a factor of 2. For example, the amount of light allowed through the lens at a setting of 2 is twice the amount allowed through the lens at a setting of 2.8. The standard diaphragm settings found on most lenses are 2, 2.8, 4, 5.6, 8, 11, 16, 22, and so on. The smallest lens opening on a lens whose f-stops end in 22 is, in fact, 22.

The second exposure control factor is the shutter, a mechanical device that acts as a gate, controlling the duration of time that light is allowed to pass through the lens and fall on th film.

Two types of shutters are in general use. The leaf type, like the diaphragm, is made up of a number of thin metal blades that are opened and closed either by a spring-driven clockwork mechanism, or--in many recent models--by an electromechanical device. Shutters of this type usually have a maximum speed of 1/500th of a second.

The focal-plane shutter in modern cameras usually consists of two pieces of rubberized fabric that move across the focal plane. The spacing between the fabric edges and the speed of transit determine the effective shutter speed. Some recent models use ultra thin pieces of titanium instead of fabric. Shutters of this type are capable of very high speeds, in some cases 1/4,000th of a second. The entire shutter mechanism is independent of the optical system, and it is therefore ideal for cameras with interchangeable lenses.

Many professional photographers still use exposure meters. These are instruments that measure light intensity and indicate what aperture and shutter speed are appropriate to the film type used, under prevailing light conditions. Completely automatic exposure control is now virtually standard on all snapshot cameras, although many new professional instruments offer an automatic system that permits the photographer to retain a great deal of individual control.

On nonreflex cameras a selenium (a toxic nonmetallic element related to sulfur) cell mounted adjacent to the lens measures the incoming light and selects a combination of lens aperture and shutter speed that will produce a negative of good quality.

Single-lens reflex cameras without exception are fitted with through-the-lens metering systems (TTLs) that offer the ultimate in automated control of exposure. A light-sensing cell is located in the optical path inside the camera and gives an extremely accurate reading of the prevailing light conditions. The information is processed by an electronic circuit built into the camera, and the aperture and shutter speed are set accordingly.

For the photographer, the view finder defines the area covered by whatever lens is in use on the camera. The most primitive type is a simple wire frame mounted just over the lens. Proper eye position is determined by a vertical post mounted at the rear of the camera. The view seen through the frame with the post in the center is equal to the area covered by the lens.

The type of viewfinder in most frequent use today is actually a reversed telescope on all cameras except single and twin-lens cameras. On a typical high-grade 35mm camera with interchangeable lenses, a bright line in the viewfinder outlines the area covered by the lens in use, and changes size automatically to correspond with the lenses of different focal lengths.

In a single-lens reflex camera the image focused by the camera lens is reflected by a mirror onto a ground-glass screen, usually through a special prism arrangement. Twin-lens reflex cameras have two coupled lenses; one of the acts as a viewfinder and, like the single-lens reflex, reflects the image it sees on a ground-glass screen.

On adjustable-lens cameras, a sharp picture requires accurate positioning of the lens system. Although its use has declined sharply, the optical-coupled range finder is one of the best methods of achieving good focus quickly. If the camera is out of focus, the user sees a double image in a portion of the viewfinder field. Focusing the lens brings the two images closer together, until--as the lens moves into focus--they are perfectly aligned.

In the single- and twin-lens reflex cameras, the image is visually focused on the ground glass in the viewfinder. Ground glass is used, wether or not the camera is fitter with a prism system for eye-level viewing. Because of the very slight distance between the picture-taking lens and the viewfinder lens in a twin-lens reflex camera, in close-ups the view seen by the photographer does not precisely match the view focused on the film. This very slight difference is called "parallax," and there are various devices available to correct it.

Many modern cameras used by casual photographers (as opposed to professional) are fitted with automatic focusing systems. There are two general types: active and passive. In the active system, a circuit so elaborate that it is actually a complete miniature computer sends out an infrared beam. This beam bounces off the photographic subject and is reflected back to the camera. By electronically measuring the angle of the bean, the distance to the subject can be determined A servomotor than adjusts the lens appropriately. The passive system works on the principle that an in-focus subject will show more contrast that an out-of-focus subject. A CCD (charge-coupled device) light sensor mounted behind the lens will search out the point of greatest contrast and set the lens. Single-lens reflex cameras often use this type of automatic focusing.