The coverage of a lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 is the size of the image

Image

An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...

 it can produce, measured as the diameter of the image circle

Image circle

The image circle, or circle of illumination, of a lens is the circular area in the image plane formed by the cone of light transmitted by the lens . Within this circle is the smaller circle for which image definition is acceptable, the circle of good definition ; however, some authors make no...

 produced by the optics. In a camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

 system the coverage must be large enough to cover the sensor or film used. In a view camera

View camera

The view camera is a type of camera first developed in the era of the Daguerreotype and still in use today, though with many refinements. It comprises a flexible bellows which forms a light-tight seal between two adjustable standards, one of which holds a lens, and the other a viewfinder or a...

 system, because the film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 plane can be moved independently of the lens plane, the lens must have enough coverage to cover the film while it is offset from the center of the coverage area. Inadequate coverage can lead to vignetting

Vignetting

In photography and optics, vignetting  is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic...

.