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A simple portable optical instrument that consists essentially of two short telescopes mounted side by side and held to both eyes; the optical system produces an erect image. Binoculars have a given combination of magnification × aperture (in mm), e.g. 8 × 40 or 10 × 50, and a wide field of view.



an optical device consisting of two parallel telescopes mounted on a single frame, used for viewing distant objects with two eyes.

There are two types of binoculars. The first type, Galileo binoculars, have an objective with a positive focal distance and an eyepiece with a negative focal distance. They render objects directly and have high aperture ratio. However, owing to the small field of vision (which is particularly significant at large magnifications), these binoculars are manufactured only with a low magnification—from 2.5-to 4-power. Second are prism binoculars. The use in binoculars of comparatively long telescopes, which produce an upside-down image, led to the introduction of an inverting system that also shortens the device. Prism binoculars with a Malofeev-Porro prism of the first type as an inverting system are the most popular. Such binoculars have several advantages : large field of vision, increased stereoscopic quality, and the possibility of using a grid in the focal plane of the objective to determine angular distances between objects and distances in general.

Binoculars, like telescopes, are classified according to magnification, angle of the visual field and diameter of the entry pupil (diameter of the objective), and plasticity (depth of field).

Binoculars can be divided into groups according to the magnification: low magnification (2- to 4-power), medium (5-to 8-power), and large (10- to 22-power). Prism binoculars are made with 3-power (opera glasses) to 22-power magnification. Binoculars with more than 10-power magnification require a stand for good viewing.

The angle of the field of vision of binoculars is inversely proportional to the magnification and directly proportional to the angle of the field of vision of the eyepiece. Types of eyepieces have been made with an angle of the field of vision of 70° or even 90° (wide-angle binoculars). The diameter of the entry pupil affects the so-called twilight number—that is, the square root of the product of the magnification of the binoculars and the diameter of the entry pupil. This number is used to assess the advantage of the binoculars for use at dusk and at night. For example, at 8-power magnification and an entry pupil diameter of 50 mm, the twilight number is Binoculars, or 20. The twilight number depends on the ratio of the brightness of the image to the brightness of the object viewed with the naked eye.

The resolving power of binoculars is determined by the diameter of the entry pupil, with the resolving power of the eye taken into account. The transmission coefficient is determined by the quality of the glass and translucence of the optics. This makes it possible to bring the transmission coefficient to 0.9 for the Galileo binocular and 0.75 for prism binoculars.

Plasticity in optics is taken to be the intensification of the stereoscopic effect—that is, intensification of the sensation of depth during viewing with binoculars for more accurate estimation of the distance to the visualized objects. The plasticity of binoculars is equal to the product of the magnification of the binoculars and the magnification of the base (the quotient from dividing the distance between the centers of the objectives by the distance between the centers of the eyepieces). The plasticity of binoculars with separated objectives ranges from 0.5 to 2; attempts are usually made to increase it. Special binoculars have been developed for persons who wear glasses. They have a greater distance between the exit pupil and the last lens of the eyepiece (11 to 12 mm in ordinary binoculars; about 18 mm in special binoculars). Therefore, whether the viewing is done with or without glasses, the visual field of the binoculars will be completely used. For persons who wear glasses, approximately one-third of the field of vision is cut off when ordinary binoculars are used owing to vignetting of the beams emerging from the binoculars.

Prism binoculars are of two kinds: central or separate focusing. Focusing is done either by moving each eyepiece separately or by moving both eyepieces together. It is also possible to move one eyepiece in relation to the other in case of unequal ametropia of the eyes. The second method is more convenient for viewing objects at various distances from the observer. Binoculars have also been constructed with so-called pancratic adjustment of magnification, which permits continuous change in magnification (for example, 7-to 12-power).


Optika ν voennom dele: Sb. statei, 3rd ed., vol. 2. Edited by S. I. Vavilov and M. V. Savost’ianova. Moscow-Leningrad, 1948.


References in periodicals archive ?
For many experts, the rest of the field in the Kappa Smurfit Champion Hurdle will need field glasses to see which way Binocular went.
When the brute fell moments later I stood and looked up at the rock with my field glasses.
Without that key, the ship's lookouts did not have access to the field glasses commonly used during those pre-sonar days to spot icebergs.
Two other collected lots being sold on Wednesday are a pair of Great War leather-bound German field glasses presented to Pte Miles following his VC heroism (pounds 50 - pounds 100) and a quantity of paperwork with autographs of many VC holders and some of the stage and screen celebrities entertaining them at the 1956 VC Centenary celebrations in London (pounds 150 - pounds 200).
The unidentified object -estimated to be several miles in diameter -was examined through telescopes and powerful field glasses,and was described as globular, with four tube-like appendages protruding from it.
The day after D-Day, Meyer mounted the tower in the Abbaye and through field glasses watched the North Nova Scotia Highlanders and the Sherbrook Fusiliers race towards his concealed tanks in the fields of St.
Bogdan watched through his field glasses to see how his courage functioned in battle, and saw him under shrapnel and annihilating shellfire, only sometimes stooping slightly; he even stepped back from the firing line and filled his fancy pipe, though he didn't light it.
As Santa Anna occupied San Antonio and trained his field glasses on the Alamo, he saw a red, white, and green Mexican flag on which was written the legend, "1824"--a reminder of the Mexican constitution of 1824, under which Texians had been promised statehood.
A couple dozen human observers huddle in a nearby blind, field glasses and telephoto lenses at the ready The late-afternoon sun paints the tallgrass bright yellow Perfect light, perfect stillness.
Far below the ruin, using my Zeiss Field glasses, I made sketches: remembrances Of the world.
You would have needed a pair of Rommel's field glasses to spot his speck in the distance.
Costa Rica's newfound eco-consciousness has spawned a whole host of private entrepreneurial preserves, like the Quetzal Farm in mountainous rural Los Santos, where the brilliant red-breasted, green-bodied quetzal bird - considered by many to be the most beautiful in the world can be glimpsed through field glasses, calmly preening its feathers.