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How do binoculars work?

Updated on May 28, 2009

Binoculars are an optical instrument used to magnify objects at a distance and designed for use with both eyes. Binoculars consist of two telescopes that are hinged together. Some types have prisms in their optical systems, while others are nonprismatic. Small nonprismatic instruments, are called opera glasses or theater glasses. Larger, more powerful nonprismatic instruments are usually called field glasses.

Binoculars are optical instruments designed for use with both eyes. Opera glasses and cheap field glasses are simple binocular telescopes. Each eye tube incorporates a convergent objective and a divergent eyepiece lens. An adjustment knob permits the eyepiece and objective lenses to be moved closer or farther apart for focusing. This simple Galilean-type instrument is limited by optical defects to a magnification of about four times.

Far more effective are prism binoculars. They have convergent objective and eyepiece lens systems which give better overall magnification. Alone they would produce an inverted reversed image. A pair of prisms is therefore incorporated which, by means of four total internal reflections, correct these effects and produces a normal image. The bending of the light rays by the prisms effectively increases the optical path through the instrument and permits greater magnification. The use of the prisms also enables the objective lenses to be set farther apart laterally than the eyepieces, whose position are naturally determined by the distance between the eyes. This greatly improves stereoscopic vision.

The magnification of common types of binoculars is between six and eight times. Binoculars with greater magnification need to be supported with a tripod so that a steady image is obtained. Binoculars are commonly described according to their magnification, and the diameter of the objective lenses in millimetres. A pair of 8 X 50 binoculars therefore provides a magnification of eight times and has 50 mm objectives. The larger the diameter of the objectives, the greater is the light-gathering power of the instrument, and the brighter the image.

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