Light-gathering powerThe ability of a pair of binoculars to pick up sufficient light for the object being viewed to be seen clearly, especially in poor light. It is also called the 'brightness factor'. For a given instrument this figure is obtained by dividing the diameter of the object glass (in millimetres) by the magnification, so that for a pair of 8 X 40 binoculars the value is 40 ÷ 8 = 5. The higher this figure the more useful the instrument is likely to be for birdwatching, and those with a light-gathering power of less than four are generally not recommended. An alternative measure of performance is the twilight factor.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver