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Binoculars are a pair of identical 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 sizes vary widely from opera glasses to large pedestal mounted military models.
Unlike a (monocular) telescope, binoculars give users a three-dimensional image. For nearer objects the two views, presented to each of the viewer's eyes from slightly different viewpoints, produce a merged view with an impression of depth. There is no need to close or obstruct one eye to avoid confusion, as is common with monocular telescopes. The use of both eyes also significantly increases the perceived visual acuity (resolution), even at greater distances where depth perception is not apparent.
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 not widely used anymore, but is still used in certain low cost models and in opera - or theater glasses.
An improved image and higher magnification can be achieved in a construction binoculars employing Keplerian optics, where the image formed by the objective lens is viewed through a positive eyepiece lens (ocular). This configuration has the disadvantage that the image is inverted. There are different ways of correcting these disadvantages.
Porro prism binoculars are named after Italian optician Ignazio Porro, who patented this image erecting system in 1854 and later refined by makers like Carl Zeiss in the 1890s. Binoculars of this type use a Porro prism in a double prism Z-shaped configuration to erect the image. This feature results in binoculars that are wide, with objective lenses that are well separated but offset from the eyepieces. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal lenght of the objective and wider spacing of the objectives gives a better sensation of depth.
Binoculars using roof prisms may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse. Most roof prism binoculars use either the Abbe-Koenig prism (named after Ernst Karl Abbe and Albert Koenig and patented by Carl Zeiss in 1905) or Schmidt-Pechan prism (invented in 1899) designs to erect the image and fold the optical path. They have objective lenses that are approximately in line with the eyepieces.
Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms. Traditionally, porro prism binoculars offered better quality viewing as the layout of the internal prisms allowed more light through and therefore provided a brighter image. Over the years, advances in lens and prism coatings mean that there is now little discernible difference between porro and roof prisms in terms of the light transmission. This combined with the more compact nature of roof prism binoculars means that it is this design which is favoured by most binocular users and manufacturers.