a prism that has five faces, a pair of which are at 90° to each other; a ray entering one of the pair emerges from the other at an angle of 90° to its original direction: used especially in single-lens reflex cameras to reverse images laterally and reflect them to the viewfinder.
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British Dictionary definitions for pentaprism
a five-sided prism that deviates light from any direction through an angle of 90°, typically used in single-lens reflex cameras between lens and viewfinder to present the image the right way round
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