Twilight factorA measure of the visual performance of binoculars or telescopes in poor light conditions. The factor is the square root of the product of the object lens (in mm) multiplied by the magnification, so that a 10 x 40 instrument has a twilight factor which is the square root of 400 which equals 20.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver