Binoculars, optical instruments consisting of two small telescopes mounted side by side, one for each eye, and a focusing mechanism. By having a lens system for each eye, these instruments provide three-dimensional viewing. They also generally provide a wide field of view. The term field glasses is sometimes used as a synonym for binoculars. Generally, however, a distinction is made: binoculars have prisms in the lens system of each telescope and field glasses do not. Opera glasses (or theater glasses) are field glasses of very low magnifying power.

The prisms of binoculars, as shown in the illustration, lengthen the light path between the objective lens and the eyepiece—thereby increasing the magnification the lens system can provide—without increasing the length of the binoculars. Porro-prism binoculars, the most common type, use an offset prism arrangement invented by Ignazio Porro, a 19th-century Italian physicist. Roof-prism binoculars are more compact because they use prisms with complex shapes to eliminate the offset arrangement; however, they are more difficult to manufacture.

Binoculars are typically designed to magnify the image of an object 6 to 12 times. Binoculars are rated both by the magnification they produce and the diameter (in millimeters) of the objective lens. Thus 6 x 30 binoculars magnify 6 times and have objective lenses 30 millimeters in diameter. The larger the objective lens, the greater its ability to gather light.