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 (Ray 2000, 49). Within this circle is the smaller circle for which image definition is acceptable, the circle of good definition (Ray 2000, 49); however, some authors (Adams 1980, 54) make no distinction between the two. The covering power or coverage of a lens is often used interchangeably with image circle or circle of good definition (Adams 1980, 54; Ray 2002, 135), and relates to the size of the image format
Film format
A film format is a technical definition of a set of standard characteristics regarding image capture on photographic film, for either stills or movies. It can also apply to projected film, either slides or movies. The primary characteristic of a film format is its size and shape.In the case of...

 for which the lens provides acceptable definition; the covering power is sometimes given as an angle of view.

A lens to be used on a camera that provides movements must have an image circle larger than the size of the image format (Adams 1980, 54). To avoid 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...

, a photographer using 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...

 must ensure that the area remains within the image circle (Adams 1980, 56–57; 151–52; 157–61); a tilt/shift lens or perspective-control lens used on a small- or medium-format camera usually has mechanical limitations that keep the image area within the image circle.

Further reading

  • Langford, Michael J. Basic Photography, 3rd ed, 63–64. Garden City, NY: Amphoto, 1973. ISBN 0-8174-0640-9
  • Ray, Sidney F. Photographic Lenses and Optics, 125. Oxford: Focal Press, 1994. ISBN 0-240-51387-8
  • Stroebel, Leslie. View Camera Technique, 3rd ed, 62–67. London: Focal Press, 1976. ISBN 0-240-50901-3
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