The lens is the eye of the camera. Its function is to bring light from the subject into focus on the film. A camera can have a single lens or a complex set of lenses. Together with the shutter, the lens controls the amount of light that enters the camera.
The modern camera’s predecessor, the camera obscura, consisted of a simple pinhole in the side of a room or box. In the 17th century people discovered they could produce a brighter, sharper image by fitting a camera obscura with a convex (outward-curving) lens. The first such lens came from a pair of eyeglasses. Over the next 300 years, interest in telescopes and microscopes led to the development of better and brighter lenses.
With the invention of photography in the 19th century, the need for camera-specific lenses increased, leading to rapid developments in the field of lens making. These developments took place along two fronts: The first was the invention of new types of glass that refracted light more effectively; and the second was the discovery of ways to combine several pieces of glass, or elements, to control optical distortion.
Quality modern lenses are made of many individual elements of ground and polished glass (6 to 14 elements is common). These elements, each of a different shape and purpose, are cemented into groups; each group is then assembled in what is called a lens barrel. On a manually controlled camera, the lens barrel incorporates an aperture ring and a focusing ring. By turning the aperture ring, the photographer adjusts the opening of the lens diaphragm, which determines how much light reaches the film. The focusing ring is used to focus the image on the film plane by changing the distance between the element groups.
Source: Microsoft ® Encarta ® 2006.
