For an equisolid angle projection (typical of full-frame fisheyes), the angle of view is double , the angle from the optical axis, and the resulting formula is , where which comes from solving the mapping function for ; Dyxum, Gustavo Orensztajn
Levett, Gray (1998). "The Nikon Legend - Part LI"(PDF). The Grays of Westminster Gazette. No. 52. London, England: Grays of Westminster. p. 8. Retrieved 8 November 2018.
GB Application 225398, Robin Hill, "Improvements in photographic lenses", published 4 December 1924, assigned to R & J Beck Ltd.
DE patennt 620538, Hans Schulz, "Abänderung eines Weitwinkelobjektivs (Modification of a wide-angle lens)", issued 23 October 1935, assigned to Allgemeine Elektricitäts-Gesellschaft
US patent 2247068, Robert Richter, "Anastigmatic photographic objective", issued 24 June 1941, assigned to Carl Zeiss, Jena
DE Grant 672393, Willi Merté, "Lichtbildlinse (Photo lens)", issued 3 October 1935, assigned to Carl Zeiss SMT GmbH
US patent 2126126, Willi Merté, "Photographic objective", issued 9 August 1938, assigned to Carl Zeiss SMT GmbH
US Grant 3524697, Isshiki Masaki & Matsuki Keiji, "Achromatic super wide-angle lens", issued 18 August 1970, assigned to Nikon Corp.
US Grant 3589798, Ogura Toshinobu, "Wide-angle lens system with corrected lateral aberration", issued 29 June 1971, assigned to Minolta Camera Kabushiki Kaisha