Coin operated binoculars, the Tower Optical Company, New York, Top of the Rock, Rockefeller Rockefeller Center Empire State bld.
Binoculars to be used to view objects that are not at a fixed distance must have a focusing arrangement which changes the distance between ocular and objective lenses. Traditionally, two different arrangements have been used to provide focus. Binoculars with "independent focus" require the two telescopes to be focused independently by adjusting each eyepiece. Binoculars designed for heavy field use, such as military applications, traditionally have used independent focusing. Because general users find it more convenient to focus both tubes with one adjustment action, a second type of binoculars incorporates "central focusing", which involves rotation of a central focusing wheel to adjust both tubes together. In addition, one of the two eyepieces can be further adjusted to compensate for differences between the viewer's eyes (usually by rotating the eyepiece in its mount). Because the focal change effected by the adjustable eyepiece can be measured in the customary unit of refractive power, the diopter, the adjustable eyepiece itself is often called a "diopter". Once this adjustment has been made for a given viewer, the binoculars can be refocused on an object at a different distance by using the focusing wheel to move both tubes together without eyepiece readjustment. Most modern binoculars are also adjustable via a hinged construction that enables the distance between the two telescope halves to be adjusted to accommodate viewers with different eye separation. Most are optimized for the interpupillary distance (typically 56mm) for adults. There are "focus-free" or "fixed-focus" binoculars that have no focusing mechanism. They are designed to have a fixed depth of field from a relatively close distance to infinity, having a large hyperfocal distance. These are considered to be compromise designs, suited for convenience, but not well suited for work that falls outside their designed.
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Photo credit: © Steve Welsh / Alamy / Afripics
License: Royalty Free
Model Released: No
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