Binoculars are lens assemblies used for magnifying and viewing distant objects stereoscopically. They are typically portable, hand-held assemblies used for bird-watching, astronomy, surveillance and other viewing applications. Because binoculars include two side-by-side tubes fitted with lenses pointed in the same direction, they provide a three dimensional image that allows the viewer to accurately estimate distances. Heavy-duty devices for military and defense applications are tripod-mounted.
Prismatic binoculars are constructed with eyepiece lenses, objective lenses, and a series of prisms to shorten the optical path. A small wheel mounted in the center between the two tubes focuses the image. Another adjustment usually in one of the eyepieces allows individuals to match the distance between their eyes.
The diameter of the objective lenses determines the resolving power of the binoculars. The ratio between the focal lengths of the objective lenses and the eyepiece lenses determines the magnifying power of the binoculars. These measurements are typically used to differentiate one set of binoculars from another. For example, a 7 x 50 device has a magnifying power of 7 with an objective lens in one of the binocular tubes that is 50 millimeters in diameter. Other parameters to consider include field of view, which is determined by the construction of the binoculars, weight, type of prisms used, refractive index, and other parameters for special use. Special use binoculars include those for use in water or at night, or those equipped with photographic equipment. Digital camera binoculars come fitted with a digital camera for taking high quality photographs.
Night vision binoculars are used to detect visible or infrared radiation at night or in dark settings by using built-in infrared illuminators for thermal sensing. Infrared binoculars are frequently used by the military or for observing wildlife or exploring caves. Binoculars for marine or boating applications are typically water resistant or waterproof. Waterproof binoculars are also typically filled with nitrogen to protect the internal optics from damage.