Eye cupA soft rubber ring on the Eyepiece of a pair of binoculars which can be folded back so that a spectacle wearer can place his eye near enough to the lens to obtain the same field of view as anyone else. Those binoculars which have this facility may be designated by the letter 'B', for example '10 x 40B'.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver