Binoculars

Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars 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: each eyepiece presents a slightly different image to each of the viewer's eyes and the parallax allows the visual cortex to generate an impression of depth. 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 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. The Galilean design is also used in low magnification binocular surgical and jewelers' loupes because they can be very short and produce an upright image without extra or unusual erecting optics, reducing expense and overall weight. They also have large exit pupils, making centering less critical, and the narrow field of view works well in those applications. These are typically mounted on an eyeglass frame or custom-fit onto eyeglasses. An improved image and higher magnification are achieved in binoculars employing Keplerian optics, where the image formed by the objective lens is viewed through a positive eyepiece lens (ocular). Since the Keplerian configuration produces an inverted image, different methods are used to turn the image the right way up. In aprismatic binoculars with Keplerian optics (which were sometimes called "twin telescopes"), each tube has one or two additional lenses (relay lens) between the objective and the eyepiece.