Binoculars or field glasses are two telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision ) when viewing distant objects. 1738, "involving both eyes," earlier "having two eyes" (1713), from French binoculaire, from Latin bini "two by two, twofold, two apiece" (see binary ) + ocularis "of the eye," from oculus "eye" (see eye (n.)). The double-tubed telescopic instrument (1871, short for binocular glass) earlier was called a binocle Related: Binocularity.
Binoculars are often specified by a set of numbers such as 7×35 or 8×40, the first number indicates the strength of magnification (how many times closer the subject is to you, 5 times closer, 7 times closer, 10 times closer) and the second number is the size of the objective lens measured in millimeters going across the lens.
This feature results in binoculars that are wide, with objective lenses that are well separated but offset from the eyepieces Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective and wider spacing of the objectives gives a better sensation of depth.
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