a hemispherical plano-convex lens for photographing in a full 180° in all directions in front of the camera, creating a circular image having an increasing amount of distortion from the center to the periphery.
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British Dictionary definitions for fisheye lens
photog a lens of small focal length, having a highly curved protruding front element, that covers an angle of view of almost 180°. It yields a circular image having considerable linear distortion
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012