A lens is a lens system of a camera that determines the incidence of light and the focal length. Such a system consists of several lenses through which the subject is projected onto the film or image sensor at a fixed or variable image scale.
Compact cameras with fixed lens systems have a fixed focal length. The situation is different for cameras with interchangeable lenses, where the focal length of the lenses can be adjusted to the subject being photographed. In addition to the normal lens, whose focal length corresponds to the human angle of view, there are the telephoto lens, the wide-angle lens and the zoom lens with a variable focal length.
The incidence of light through the lens system is controlled by the aperture. At the moment of exposure, the shutter opens and closes the aperture for the exposure time, allowing light to fall on the film or image sensor. The exposure times are determined by the shutter speed, Shutter Speed, and range from 1 s to 1/500 s. The aperture is determined by the f-number. It can be larger or smaller and, together with the exposure time, determines the incidence of light on the light-sensitive element.
The maximum incidence of light occurs at the largest possible aperture. This is specified by the aperture ratio, the reciprocal of the f-number, and corresponds to the luminous intensity of the lens.