Binoculars telescopes, or Binocular, (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, permitting 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 versions.
FOTAR Binoculars Unlike a monocular telescope, Field glasses give people 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 shut or obstruct one eye to avoid confusion, as is usual with monocular telescopes. A conventional Porro prism Binocular concept A conventional Porro prism Binocular concept Contents
FOTAR Binoculars Almost from the invention of the telescope in the 17th century the advantages of mounting two of them next to each other for Binocular vision look atms to have been explored[1]. Most early Binocular 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 low models and in "opera glasses" or theater glasses.
FOTAR Binoculars Named after Italian optician Ignazio Porro who patented this image erecting system in Binocular telescopes, or binoculars, (also known as field glasses) are two identical or mirror-symmetrical telescopes mounted next to each other and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binoculars vision) when watching distant objects. Most are sized to be held using both hands, although there are much bigger versions.
FOTAR Binocular Unlike a monocular telescope, Binocular give 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. A typical Porro prism Field glasses concept A conventional Porro prism Binoculars concept Contents
FOTAR Field glasses Almost from the invention of the telescope in the 17th century the advantages of mounting two of them next to each other for Field glasses vision look atms to have been explored[1]. Most early Binoculars used Galilean optics; that is they used a convex objective and a concave eyepiece lens. The Galilean concept 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 low cost models and in "opera glasses" or theater glasses.
FOTAR Field glasses 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[1], Field glasses of this type use a Porro prism in a double prism Z-shaped configuration to erect the image. This feature results in Field glasses that are wide, with objective lenses that are well separated but offset from the eyepieces. Porro prism designs have the added feature of folding the optical path so that the physical length of the Binocular is less than the focal length of the objective and wider spacing of the objectives gives better sensation of depth.
FOTAR Field glasses Binocular using Roof prisms may have appeared as early as the 1880s in a design by Achille Victor Emile Daubresse[2] [3]. Most roof prism Field glasses use either the Abbe-Koenig prism (named after Ernst Karl Abbe and Albert Koenig and patented by Carl Zeiss in 1905)[4] or Schmidt-Pechan prism (invented in 1899) designs to erect the image and fold the optical path. They have objective lenses that are more or less in line with the eyepieces.
FOTAR Binocular 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 Field glasses will inherently produce a brighter image than roof-prism Binoculars of the same magnification, objective size, and optical quality, because the roof-prism design utilize 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 concept requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms Field glasses sometimes need their prism sets to be re-aligned to bring them into collimation. The fixed alignment in roof-prism designs means the Field glasses normally won't need re-collimation[5].
FOTAR Field glasses Parameters shown on the prism cover plate describing a 7 power magnification Field glasses with a 50 mm Objective diameter and a 372-foot (113 m) Field of view at 1,000 yards (1,000 m). Parameters shown on the prism cover plate describing a 7 power magnification Field glasses with a 50 mm Objective diameter and a 372-foot (113 m) Field of view at 1,000 yards (1,000 m).
FOTAR Field glasses Field glasses are usually designed for the specific situation for which they are intended. Those different designs create certain optical Details (some of which may be shown on the prism cover plate of the Field glasses). Those Parameters are:
FOTAR Field glasses The ratio of the focal length of the eyepiece divided into the focal length of the objective gives the linear magnifying power of Binoculars (sometimes expressed as "diameters"). A magnification of factor 7, for example, produces an image as if one were 7 times closer to the object. The amount of magnification depends upon the situation the Binocular are designed for. Hand-held Binoculars have lower magnifications so they will be less susceptible to shaking. A bigger magnification leads to a smaller field of view.
FOTAR Binocular The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
FOTAR Binoculars It is customary to categorize Field glasses by the magnification × the objective diameter; e.g. 7×50.
FOTAR Field glasses The field of view of Binoculars is determined by its optical concept. It is usually notated in a linear value, such as how many feet (meters) in width will be seen at 1,000 yards (or 1,000 m), or in an angular value of how many degrees can be viewed.
FOTAR Binoculars Binocular concentrate the light gathered by the objective into a beam, the exit pupil, whose diameter is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye—about 7 mm, reducing with age. Light collected by a larger exit pupil is wasted. For daytime use an exit pupil of 3 mm—matching the eye's contracted pupil—is sufficient. However, a bigger exit pupil makes alignment of the eye easier and avoids dark vignetting intruding from the edges.
FOTAR Field glasses Eye relief is the distance from the rear eyepiece lens to where the image is formed. It determines the distance the observer must position his or her eye behind the eyepiece in order to look at an unvignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Binocular may have eye relief ranging from few millimeters to 2.5 centimeters or more. Eye relief can be particularly important for eyeglass wearers. The eye of an eyeglass wearer is conventionally further from the eye piece which necessitates a longer eye relief in order to still look at the entire field of view. Binoculars with short eye relief can also be hard to use in instancEs where it is difficult to hold them steady.
FOTAR Binocular Since Field glasses may have 16 air-to-glass surfaces, with light lost at every surface, optical coatings can significantly affect their image quality. When light strikes an interface between two materials of different refractive index (e.g., at an air-glass interface), some of the light is transmitted, some reflected. In any sort of image-forming optical instrument (telescope, camera, microscope, etc.), ideally no light should be reflected; instead of forming an image, light which reaches the viewer after being reflected is distributed in the field of view, and reduces the contrast between the true image and the background. Reflection can be reduced, but not done away with, by applying optical coatings to interfaces. Each time light enters or leaves a piece of glass; about 5% is reflected back. This "lost" light bounces around inside the Field glasses, making the image hazy and hard to see. Lens coatings effectively lower reflection losses, which finally results in a brighter and sharper image. For example, 8x40 Field glasses with good optical coatings will yield a brighter image than uncoated 8x50 Binoculars. Light can also be reflected from the interior of the instrument, but it is simple to minimize this to negligible proportions. Contrast is also improved by good coating due to the partial elimination of internal reflections.
FOTAR Binocular A classic lens-coating material is magnesium fluoride; it reduces reflections from 5% to 1%. Modern lens coatings consist of complicated multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors. For roof-prisms, anti-phase shifting coatings are sometimes used which significantly improve contrast. The presence of a coating is conventionally denoted on Binoculars by the following terms:
* coated optics: one or more surfaces coated. * fully coated: all air-to-glass surfaces coated. Plastic lenses, however, if used, may not be coated. * multi-coated: one or more surfaces are multi-layer coated. * fully multi-coated: all air-to-glass surfaces are multi-layer coated.
Phase-corrected prism coating and dielectric prism coating are recent (in 2005) effective techniques for reducing reflections.
FOTAR Binoculars Field glasses to be used to view objects that are not at a fixed distance must have a focusing arrangement. Traditionally, two different arrangements have been used to provide focus. Field glasses with "independent focus" need the two telescopes to be focused independently by adjusting each eyepiece, thereby changing the distance between ocular and objective lenses. Field glasses designed for heavy field use, such as military applications, traditionally have used independent focusing. Because general people find it more convenient to focus both tubes with one change in setting action, a second type of Binocular incorporates "central focusing", which involves rotation of a central focusing wheel. In addition, one of the two eyepieces can be further adjusted to compensate for differences between the viewer's eyes (usually by rotating the eyepiece in its mount). This is known as a diopter. Once this adjustment has been made for a given viewer, the Binoculars can be refocused on an object at a different distance by using the focusing wheel to move both tubes together without eyepiece readjustment.
FOTAR Binoculars There are also "focus-free" or "fixed-focus" Binocular. They have a depth of field from a relatively large closest distance to infinity, and perform exactly the same as a focusing model of the same optical quality (or lack of it) focused on the middle distance.
Zoom Binocular, while in principle a good idea, are generally considered not to perform very well.[6][7]
Most modern Field glasses have hinged-telescope construction that enables the distance between eyepieces to be adjusted to accommodate viewers with different eye separation. This adjustment feature is lacking on many older Binoculars.
FOTAR Binocular Shake can be much reduced, and higher magnifications used, with Binoculars using image-stabilization technology. Parts of the instrument which change the position of the image may be held steady by powered gyroscopes or by powered mechanisms driven by gyroscopic or inertial detectors, or may be mounted in such a way as to oppose and dampen sudden movement. Stabilization may be enabled or disabled by the user as needd. These techniques allow Field glasses up to 20× to be hand-held, and much improve the image stability of lower-power instruments. There are some disadvantages: the image may not be quite as good as the best unstabilized Field glasses when tripod-mounted, stabilized Field glasses also tend to be more expensive and heavier than similarly specified non-stabilised Binocular.
FOTAR Field glasses Well-collimated Binoculars, when viewed through human eyes and processed by a human brain, should produce a single circular, apparently three-dimensional image, with no visible indication that one is actually viewing two distinct images from slightly different viewpoints. Departure from the ideal will cause, at best, vague discomfort and visual fatigue, but the perceived field of view will be shut to circular anyway. The cinematic convention used to represent a view through Binocular as two circles partially overlapping in a figure-of-eight shape is not true to life.
FOTAR Binocular Misalignment is remedied by small movements to the prisms, often by turning screws accessible without opening the Binocular, or by adjusting the position of the objective via eccentric rings built into the objective cell. Alignment is usually performed by a professional although instructions for checking Field glasses for collimation errors and for collimating them can be found on the Internet.
FOTAR Binoculars Hand-held Binoculars range from small 3x10 Galilean opera glasses, used in theaters, to glasses with 7 to 12 diameters magnification and 30 to 50 mm objectives for typical outdoor use. Porro prism models predominate although bird watchers and hunters tend to prefer, and are prepared to pay for, the lighter but more expensive roof-prism models.
Many tourist attractions have installed pedestal-mounted, coin-operated Binocular to allow tourists to obtain a closer view of the attraction. In the United World, 20 pence often gives a couple of minutes of operation, and in the United States, one or two quarters gives between one-and-a-half to two-and-a-half minutes.
FOTAR Field glasses Binoculars have a long history of military use. Galilean designs were widely used up to the end of the 19th century when they gave way to porro prism types. Binocular constructed for general military use tend to be more heavily ruggedized than their civilian counterparts. They generally avoid more fragile center focus arrangements in favor of independent focus. Prism sets in military Field glasses may have redundant aluminized coatings on their prism sets to guarantee they don’t lose their reflective qualities if they get wet. Military Binoculars of the cold war era were sometimes fitted with passive sensors that detected active IR emissions, while modern ones usually are fitted with filters blocking laser beams. Further, Field glasses designed for military usage may include a stadiametric reticle in one ocular in order to facilitate range estimation.
FOTAR Binocular There are Binoculars designed specifically for civilian and military use at sea. Hand held models will be 5× to 7× but with very large prism sets combined with eyepieces designed to give generous eye relief. This optical combination prevents the image vignetting or going dark when the Binocular are pitching and vibrating relative to the viewer's eye. Large, high-magnification, models with large objectives are also used in fixed mountings.
Very large Binocular naval rangefinders (up to 15 meters separation of the two objective lenses, weight 10 tons, for ranging World War II naval gun targets 25 km away) have been used, although late-20th century technology made this application redundant.
FOTAR Binoculars Field glasses are widely used by amateur astronomers; their wide field of view making them useful for comet and supernova look atking (giant Field glasses) and general observation (portable Field glasses). The Galilean moons of Jupiter, Ceres, Neptune, Pallas and Titan are invisible to the naked eye but can readily be seen with Binoculars. Although technically visible unaided in pollution-free skies, Uranus and Vesta require Field glasses for practical observation. 10×50 Field glasses are limited to a magnitude of around +9.5, which means asteroids like Interamnia, Davida, Europa and, except under exceptional conditions Hygiea, are too faint to be seen with Binocular. Likewise too faint to be look atn with Binoculars are all moons except the Galileans and Titan, and the dwarf planets Pluto and Eris.
FOTAR Binocular Of particular relevance for low-light and astronomical viewing is the ratio between magnifying power and objective lens diameter. A lower magnification facilitates a bigger field of view which is useful in viewing large deep sky objects such as the Milky Way, nebula, and galaxies, though the large exit pupil means some of the gathered light is wasted. The large exit pupil will also image the night sky background, effectively decreasing contrast, making the detection of faint objects more difficult except perhaps in remote locations with negligible light pollution. Binoculars specifically for most astronomical uses have higher magnification and a larger aperture objective (in the 70mm or 80mm range) because the diameter of the objective lens determines the faintest star that can be observed. These Binocular usually require some sort of mount
FOTAR Binoculars Much bigger Binoculars have been made by amateur telescope makers, essentially using two refracting or reflecting astronomical telescopes, with mixed results. A very large professional instrument, although not one that would normally be called Field glasses, is the Large Binoculars Telescope in Arizona, USA, which produced its "First Light" image on October 26, 2005. The LBT comprises two 8-meter reflector telescopes. While obviously not intended to be held to the eyes of a viewer, it uses two telescopes to view the same object, giving higher resolving power than a single instrument of the same light-gathering power, and permitting interferometric use.
