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Binoculars, field glasses or binocular telescopes are a pair of 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 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.
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. 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 unordinary 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 eye-glass frame or custom-fit onto eyeglasses.
An improved image and higher magnification is 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 were used to turn the image 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 ocular. These lenses are used to erect the image. The binoculars with erecting lenses have a serious disavantage: their length is too big. Such binoculars were popular in 1800s (for example, G.& S. Merz models), but became obsolete shortly after Karl Zeiss company invented improved prism binoculars in 1890s.
Optical prisms added to the design is another way to turn the image right way up, usually in a Porro prism or roof-prisms design.
Porro prism binoculars are named after Italian optician Ignazio Porro who patented this image erecting system in 1854 and later refined by makers like the Carl Zeiss company 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 length of the objective and wider spacing of the objectives gives a better sensation of depth. Thus, the size of binoculars is reduced.
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 the 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. 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 for 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 will not need re-collimation.
Binoculars are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters (some of which may be listed on the prism cover plate of the binocular). Those parameters are:
Binoculars have a focusing arrangement which changes the distance between ocular and objective lenses. Normally there are two different arrangements used to provide focus, "independent focus" and "central focusing":
There are "focus-free" or "fixed-focus" binoculars that have no focusing mechanism other than the eyepiece adjustments that are meant to be set for the user's eyes and left fixed. These are considered to be compromise designs, suited for convenience, but not well suited for work that falls outside their designed range.
Binoculars can be generally used without eyeglasses by myopic (near-sighted) or hyperopic (far-sighted) users simply by adjusting the focus a little further. Most manufacturers leave a little extra available focal-range beyond the infinity-stop/setting to account for this when focusing for infinity. People with severe astigmatism, however, may still need to use their glasses while using binoculars.
Some binoculars have adjustable magnification, zoom binoculars, intended to give the user the flexibility of having a single pair of binoculars with a wide range of magnifications, usually by moving a "zoom" lever. This is accomplished by a complex series of adjusting lenses similar to a zoom camera lens. These designs are noted to be a compromise and even a gimmick since they add bulk, complexity and fragility to the binocular. The complex optical path also leads to a narrow field of view and a large drop in brightness at high zoom. Models also have to match the magnification for both eyes throughout the zoom range and hold collimation to avoid eye strain and fatigue.
Most modern binoculars are also adjustable via a hinged construction that enables the distance between the two telescope halves to be adjusted to accommodate viewers with different eye separation or "interpupillary distance". Most are optimized for the interpupillary distance (typically 56mm) for adults.
Some binoculars use image-stabilization technology to reduce shake at higher magnifications. This is done by having a gyroscope move part of the instrument, or by powered mechanisms driven by gyroscopic or inertial detectors, or via a mount designed to oppose and damp the effect of shaking movements. Stabilization may be enabled or disabled by the user as required. These techniques allow binoculars 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 binoculars when tripod-mounted, stabilized binoculars also tend to be more expensive and heavier than similarly specified non-stabilised binoculars.
The two telescopes in binoculars are aligned in parallel (collimated), to produce a single circular, apparently three-dimensional, image. Misalignment will cause the binoculars to produce a double image. Even slight misalignment will cause vague discomfort and visual fatigue as the brain tries to combine the skewed images.
Alignment is performed by small movements to the prisms, by adjusting an internal support cell or by turning external set screws, or by adjusting the position of the objective via eccentric rings built into the objective cell. Alignment is usually done by a professional, although the externally mounted adjustment features can be accessed by the end user.
Since a typical binocular has 6 to 10 optical elements  with special characteristics and up to 16 air-to-glass surfaces, binocular manufactures use different types of optical coatings for technical reasons and to improve the image they produce.
Anti-reflective coatings reduce light lost at every optical surface through reflection at each surface. Reducing reflection via anti-reflective coatings also reduces the amount of "lost" light bouncing around inside the binocular which can make the image appear hazy (low contrast). A pair of binoculars with good optical coatings may yield a brighter image than uncoated binoculars with a larger objective lens, on account of superior light transmission through the assembly. A classic lens-coating material is magnesium fluoride, which reduces reflected light from 5% to 1%. Modern lens coatings consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors.
In binoculars with roof prisms the light path is split in two paths that reflect on either side of the roof prism ridge. One half of the light reflects from roof surface 1 to roof surface 2. The other half of the light reflects from roof surface 2 to roof surface 1. This causes the light to becomes partially polarized (due to a phenomenon called Brewster's angle). During subsequent reflections the direction of this polarization vector is changed but it is changed differently for each path in a manner similar to a Foucault pendulum. When the light following the two paths are recombined the polarization vectors of each path do not coincide. The angle between the two polarization vectors is called the phase shift, or the geometric phase, or the Berry phase. This interference between the two paths with different geometric phase results in a varying intensity distribution in the image reducing apparent contrast and resolution compared to a porro prism erecting system. These unwanted interference effects can be suppressed by vapour depositing a special dielectric coating known as a phase-correction coating or P-coating on the roof surfaces of the roof prism. This coating corrects for the difference in geometric phase between the two paths so both have effectively the same phase shift and no interference degrades the image.
Binoculars using either a Schmidt-Pechan roof prism or an Abbe-Koenig roof prism benefit from phase coatings. Porro prism binoculars do not recombine beams after following two paths with different phase and so do not benefit from a phase coating.
In binoculars with Schmidt-Pechan roof prisms, mirror coatings are added to some surfaces of the roof prism because the light is incident at one of the prism's glass-air boundaries at an angle less than the critical angle so total internal reflection does not occur. Without a mirror coating most of that light would be lost. Schmidt-Pechan roof prism use aluminium mirror coating (reflectivity of 87% to 93%) or silver mirror coating (reflectivity of 95% to 98%) is used.
In older designs silver mirror coatings were used but these coatings oxidized and lost reflectivity over time in unsealed binoculars. Aluminium mirror coatings were used in later unsealed designs because it did not tarnish even though it has a lower reflectivity than silver. Modern designs use either aluminium or silver. Silver is used in modern high-quality designs which are sealed and filled with a nitrogen or argon inert atmosphere so the silver mirror coating doesn't tarnish.
Porro prism binoculars and roof prism binoculars using the Abbe-Koenig roof prism typically do not use mirror coatings because these prisms reflect with 100% reflectivity using total internal reflection in the prism.
Dielectric coatings are used in Schmidt-Pechan roof prism to cause the prism surfaces to act as a dielectric mirror. The non-metallic dielectric reflective coating is formed from several multilayers of alternating high and low refractive index materials deposited on the roof prism's reflective surfaces. Each single multilayer reflects a narrow band of light frequencies so several multilayers, each tuned to a different color, are required to reflect white light. This multi-multilayer coating increases reflectivity from the prism surfaces by acting as a distributed Bragg reflector. A well-designed dielectric coating can provide a reflectivity of more than 99% across the visible light spectrum. This reflectivity is much improved compared to either an aluminium mirror coating (87% to 93%) or silver mirror coating (95% to 98%).
Porro prism binoculars and roof prism binoculars using the Abbe-Koenig roof prism do not use dielectric coatings because these prisms reflect with very high reflectivity using total internal reflection in the prism rather than requiring a mirror coating.
The presence of any coatings is typically denoted on binoculars by the following terms:
Many tourist attractions have installed pedestal-mounted, coin-operated binoculars to allow visitors to obtain a closer view of the attraction.
Many binoculars have range finding reticle (scale) superimposed upon the view. This scale allows the distance to the object to be estimated if the object's height is known (or estimable). The common mariner 7×50 binoculars have these scales with the angle between marks equal to 5 mil. One mil is equivalent to the angle between the top and bottom of an object one meter in height at a distance of 1000 meters.
Therefore to estimate the distance to an object that is a known height the formula is:
With the typical 5 mil scale (each mark is 5 mil), a lighthouse that is 3 marks high that is known to be 120 meters tall is 8000 meters distance.
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. Binoculars constructed for general military use tend to be more rugged than their civilian counterparts. They generally avoid fragile center focus arrangements in favor of independent focus, which also makes for easier, more effective weatherproofing. Prism sets in military binoculars may have redundant aluminized coatings on their prism sets to guarantee they don't lose their reflective qualities if they get wet.
One variant form was called "trench binoculars", a combination of binoculars and periscope, often used for artillery spotting purposes. It projected only a few inches above the parapet, thus keeping the viewer's head safely in the trench.
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 used as weapons. Further, binoculars designed for military usage may include a stadiametric reticle in one ocular in order to facilitate range estimation.
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 binoculars 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.
Binoculars are widely used by amateur astronomers; their wide field of view makes them useful for comet and supernova seeking (giant binoculars) and general observation (portable binoculars). Binoculars specifically geared towards astronomical viewing will have larger aperture objectives (in the 70 mm or 80 mm range) because the diameter of the objective lens increases the total amount of light captured, and therefore determines the faintest star that can be observed. Binoculars designed specifically for astronomical viewing (often 80 mm and larger) are sometimes designed without prisms in order to allow maximum light transmission. Such binoculars also usually have changeable eyepieces to vary magnification. Binoculars with high magnification and heavy weight usually require some sort of mount to stabilize the image. 10x is generally considered the practical limit for observation with handheld binoculars. Binoculars more powerful than 15×70 require support of some type. Much larger binoculars have been made by amateur telescope makers, essentially using two refracting or reflecting astronomical telescopes.
Of particular relevance for low-light and astronomical viewing is the ratio between magnifying power and objective lens diameter. A lower magnification facilitates a larger field of view which is useful in viewing the Milky Way and large nebulous objects (referred to as deep sky objects) such as the nebulae and galaxies. The large (typical 7 mm using 7x50) exit pupil [objective (mm)/power] of these devices results in a small portion of the gathered light not being usable by individuals whose pupils do not sufficiently dilate. For example, the pupils of those over 50 rarely dilate over 5 mm wide. The large exit pupil also collects more light from the background sky, effectively decreasing contrast, making the detection of faint objects more difficult except perhaps in remote locations with negligible light pollution. Many astronomical objects of 8 magnitude or brighter, such as the star clusters, nebulae and galaxies listed in the Messier Catalog, are readily viewed in hand-held binoculars in the 35 to 40 mm range, as are found in many households for birding, hunting, and viewing sports events. For observing smaller star clusters, nebulae, and galaxies binocular magnification is an important factor for visibility because these objects appear tiny at typical binocular magnifications.
Some open clusters, such as the bright double cluster (NGC 869 and NGC 884) in the constellation Perseus, and globular clusters, such as M13 in Hercules, are easy to spot. Among nebulae, M17 in Sagittarius and the North American nebula (NGC 7000) in Cygnus are also readily viewed. Binoculars can show a few of the wider-split binary star such as Albireo in the constellation Cygnus.
A number of solar system objects that are mostly to completely invisible to the human eye are reasonably detectable with medium-size binoculars, including larger craters on the Moon; the dim outer planets Uranus and Neptune; the inner "minor planets" Ceres, Vesta and Pallas; Saturn's largest moon Titan; and the Galilean moons of Jupiter. Although visible unaided in pollution-free skies, Uranus and Vesta require binoculars for easy detection. 10×50 binoculars are limited to an apparent magnitude of +9.5 to +11 depending on sky conditions and observer experience. Asteroids like Interamnia, Davida, Europa and, unless under exceptional conditions Hygiea, are too faint to be seen with commonly sold binoculars. Likewise too faint to be seen with most binoculars are the planetary moons except the Galileans and Titan, and the dwarf planets Pluto and Eris. Other difficult binocular targets include the phases of Venus and the rings of Saturn. Only binoculars with very high magnification, 20x or higher, are capable of discerning Saturn's rings to a recognizable extent. High-power binoculars can sometimes show one or two cloud belts on the disk of Jupiter if optics and observing conditions are sufficiently good.
|This section's factual accuracy is disputed. (September 2010)|
There are many companies that manufacturer binoculars, both past and present. They include:
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