Binoculars

Binoculars, optical instrument that magnifies distant objects. It consists of two telescopes that can be focused simultaneously by means of a thumbscrew and that are usually capable of independent adjustment for each eye. Each telescope is equipped with a small concave lens called the eyepiece, or ocular, and a large convex lens called the objective lens, or objective. Two prisms are often used in each telescope to bend the path of the light, thereby counteracting the effect of the inverted image provided by the concave eyepiece and allowing the binocular to be shorter. Depth perception increases with an increase in distance between the objects (see Vision). An opera glass is a simple binocular with inexpensive lenses that provide a limited field of vision and a small amount of magnification. Prism binoculars give greater magnification and a wider field.

Binoculars are classified by their magnifying capability and their power of collecting light, as, for example, 6 × 30, 7 × 50, or 8 × 30. The first number represents the magnification, the second, the diameter of the objective lens in millimeters. With higher magnification and larger objectives, binoculars become too heavy for use without a fixed support. Where stereoscopic vision is not important, the lighter monocular—half of a prism binocular—is sometimes used.