Depth of field or depth of focusThe distance within which the image seen through a pair of binoculars or a telescope remains reasonably sharp and in focus. Generally the higher the magnification, the further will be the nearest distance from the observer at which focusing becomes possible, so more powerful instruments usually have a reduced depth of field and a poorer close-focus performance and are less useful for watching nearby birds.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver