binoculars(redirected from field glasses)
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binocularsA 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 , 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).
REFERENCEOptika ν voennom dele: Sb. statei, 3rd ed., vol. 2. Edited by S. I. Vavilov and M. V. Savost’ianova. Moscow-Leningrad, 1948.
N. I. KUTNIAKOVA