BINOCULAR

optical instrument for providing a magnified view of distant objects, consisting of two similar telescopes, one for each eye, mounted on a single frame. A single thumbscrew may control the focus of both telescopes simultaneously, and provision may be made for focusing each separately to allow for varying characteristics in the two eyes. Prismatic binoculars In most binoculars, each telescope is provided with two prisms. The prisms reinvert the inverted image provided by the ocular, or eyepiece, of each telescope. They prescribe a folded path for the light rays, allowing a shorter overall length for the instrument. The prisms also provide better stereoscopic effectthat is, depth perception at greater distancesby allowing the two objectives (object lenses) to be set farther apart than the eyepieces. The arrangement of these prisms and the other optical components is shown in the illustration. Binoculars are commonly designated as 6 30, 7 50, or 8 30. The first number indicates the magnification and the second the diameter of the objective lens in millimetres (one inch is about 25 millimetres). This latter figure is a measure of the light-gathering power of the instrument. In applications in which depth perception is not essential, a single telescope, called a monocular, may be employed. It is essentially one-half of a pair of binoculars. Opera glasses are binoculars with simple, inexpensive lens systems and narrow fields of view and are usually made with magnifications of only 2 1/2 to 4. The lenses used in most binoculars are coated to prevent reflections.