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 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, a photographer using a view camera 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.

References

• Adams, Ansel. 1980. The Camera. The New Ansel Adams Basic Photography Series/Book 1. ed. Robert Baker. Boston: New York Graphic Society. ISBN 0-8212-1092-0
• Ray, Sidney F. 2000. The geometry of image formation. In The Manual of Photography: Photographic and Digital Imaging, 9th ed. Ed. Ralph E. Jacobson, Sidney F. Ray, Geoffrey G. Atteridge, and Norman R. Axford. Oxford: Focal Press. ISBN 0-240-51574-9
• Ray, Sidney F. 2002. Applied Photographic Optics, 3rd ed. Oxford: Focal Press ISBN 0-240-51540-4

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