Binoculars, (French binocle, from the Latin bini — pair, two, and oculus—eye), an optical instrument for viewing distant objects with both eyes, and also for measuring angles and distances. Binoculars consist of two telescopes joined so that their optical axes are parallel. The optical system of binoculars comprises a converging objective, usually a system of two cemented lenses, and an eyepiece. The main characteristics of binoculars — the magnification, the angular field of view and the resolving power — are defined in the same way as for telescopes.
The real, reduced and inverted image of a distant object, which lies in the focal plane of the objective or close to it, is viewed through the eyepiece of the binoculars as through a magnifying glass. In binoculars whose telescopes are of the Keplerian type the eyepiece is also a converging system, and the image it gives is inverted. An erect image is obtained by placing an erecting system between the objective and the eyepiece, for example a Malofeev — Porro system, which consists of two right-angle prisms with two reflecting faces set at 90° to each other (Fig. 1).
Fig. 1. Prismatic binoculars with a Malofeev — Porro erecting system. Such a system also makes it possible to shorten the overall length of the instrument considerably. Binoculars of this type are often called prismatic. Since the plane of the real intermediate image produced by the objective in a Keplerian telescope lies between the objective and the eyepiece, an angle-measuring graticule is placed in that plane in one of the telescopes, and with it angles and distances can be measured. Binoculars of this type usually have magnifications of 6, 8 and 15 times, with fields of view of 10°, 8°30` and 4° respectively. In binoculars whose telescopes are of the Galilean type (Fig. 2) the eyepiece is a diverging lens placed in front of the plane of the real image; the length of a telescope of this type is always less than the focal length of the objective, and the image obtained is virtual and erect. Fig. 2. Binoculars with telescopes of the Galilean type. Galilean telescopes are simple in construction and lose little brightness, but because of their limited field of view, which is especially noticeable at high magnifications, they are used in binoculars with magnifications from 2 to 4 times, for example in opera glasses.
Viewing through binoculars reduces eye fatigue and makes it easier to perceive objects lying at different distances from the observer, because the radius of stereoscopic vision is increased. The ability of binoculars to increase the radius of stereoscopic vision, which for the unaided eyes is 1350 m, is called plasticity. When viewing through binoculars, the radius of stereoscopic vision increases by the same factor by which the distance between the axes of the objectives exceeds the distance between the axes of the eyes. The plasticity of binoculars with widely spaced objectives reaches a value close to 2, that is, the radius of stereoscopic vision increases to 2.7 km.
The real, reduced and inverted image of a distant object, which lies in the focal plane of the objective or close to it, is viewed through the eyepiece of the binoculars as through a magnifying glass. In binoculars whose telescopes are of the Keplerian type the eyepiece is also a converging system, and the image it gives is inverted. An erect image is obtained by placing an erecting system between the objective and the eyepiece, for example a Malofeev — Porro system, which consists of two right-angle prisms with two reflecting faces set at 90° to each other (Fig. 1).
Fig. 1. Prismatic binoculars with a Malofeev — Porro erecting system. Such a system also makes it possible to shorten the overall length of the instrument considerably. Binoculars of this type are often called prismatic. Since the plane of the real intermediate image produced by the objective in a Keplerian telescope lies between the objective and the eyepiece, an angle-measuring graticule is placed in that plane in one of the telescopes, and with it angles and distances can be measured. Binoculars of this type usually have magnifications of 6, 8 and 15 times, with fields of view of 10°, 8°30` and 4° respectively. In binoculars whose telescopes are of the Galilean type (Fig. 2) the eyepiece is a diverging lens placed in front of the plane of the real image; the length of a telescope of this type is always less than the focal length of the objective, and the image obtained is virtual and erect. Fig. 2. Binoculars with telescopes of the Galilean type. Galilean telescopes are simple in construction and lose little brightness, but because of their limited field of view, which is especially noticeable at high magnifications, they are used in binoculars with magnifications from 2 to 4 times, for example in opera glasses.
Viewing through binoculars reduces eye fatigue and makes it easier to perceive objects lying at different distances from the observer, because the radius of stereoscopic vision is increased. The ability of binoculars to increase the radius of stereoscopic vision, which for the unaided eyes is 1350 m, is called plasticity. When viewing through binoculars, the radius of stereoscopic vision increases by the same factor by which the distance between the axes of the objectives exceeds the distance between the axes of the eyes. The plasticity of binoculars with widely spaced objectives reaches a value close to 2, that is, the radius of stereoscopic vision increases to 2.7 km.