Binoculars & Telescopes

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. more...

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Most are sized adequate to be held using both hands although there are much larger types.

Unlike a monocular telescope, binoculars give users a three-dimensional image: the two views, presented from slightly different viewpoints to each of the viewer's eyes, merge to produce a single perceived view with a sensation of depth, allowing distances to be estimated. Binoculars are also more comfortable for viewing, as they negate the need to close or obstruct one eye to avoid confusion. It is also easier and more comfortable to steadily hold and move a binocular than a single tube, as the two hands and the head form a steady three-point platform.

Optical design

Galilean binoculars

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 construction is still used in very cheap models and in "opera glasses" or theater glasses.

Porro prism binoculars

Named after Italian optician Ignazio Porro who patented this image erecting system in 1854 and later refined by makers like Carl Zeiss in the 1890's, 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 length of the objective and wider spacing of the objectives gives better sensation of depth.

Roof prism binoculars

Binoculars using Roof prisms may have appeared as early as the 1880s 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 are narrower, more compact, and more expensive than those that use Porro prisms. They have objective lenses that are approximately in line with the eyepieces.

Read more at Wikipedia.org