Binoculars

Binocular telescopes, or binoculars (also known as field glasses), are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most are sized to be held using both hands, although there are much larger types. Unlike a monocular telescope, a binocular gives users a three-dimensional image: the two views, presented from slightly different viewpoints to each of the viewer's eyes, produce a merged view with depth perception. There is no need to close or obstruct one eye to avoid confusion, as is usual with monocular telescopes.

Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored. Most early binoculars used Galilean optics; that is they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification. This type of construction is still used in very cheap models and in opera glasses or theater glasses.

Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms. There is also a difference in image brightness. Porro-prism binoculars will inherently produce a brighter image than roof-prism binoculars of the same magnification, objective size, and optical quality, because the roof-prism design employs silvered surfaces that reduce light transmission by 12% to 15%. Roof-prisms designs also require tighter tolerances as far as alignment of their optical elements (collimation). This adds to their expense since the design requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation. The fixed alignment in roof-prism designs means the binoculars normally won't need re-collimation.

In older binocular designs silver mirror coatings were used but these coatings oxidized and lost reflectivity over time in unsealed binoculars. Aluminum mirror coatings were used in later unsealed designs because it did not tarnish even though it has a lower reflectivity than silver. Modern binocular designs use either aluminum or silver. Silver is used in modern high-quality designs as modern binoculars are sealed and nitrogen or argon filled so the silver mirror coating doesn't tarnish in an inert atmosphere. Porro prism binoculars and roof prism binoculars using the Abbe-Koenig roof prism do not use mirror coatings because these prisms reflect with 100% reflectivity using total internal reflection in the prism.