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[[File:Binocularp.svg|thumb|300px|A typical [[Porro prism]] binoculars design]]
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[[File:Lunette-binoculaire.jpg|thumb|Binoculars, by Father Chérubin d'Orléans, 1681, [[Musée des Arts et Métiers]]]]
'''Binoculars''', '''field glasses''' or '''binocular telescopes''' are a pair of identical or [[symmetry|mirror-symmetrical]] [[optical telescope|telescope]]s 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 [[depth perception|impression of depth]].
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== Optical designs ==
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=== Galilean binoculars ===
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[[File:Fernglas(alt).JPG|thumb|Galilean binoculars]]
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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.<ref name="europa">[http://www.europa.com/~telscope/binohist.txt Europa.com]&nbsp;— The Early History of the Binocular</ref> Most early binoculars used [[Galilean telescope|Galilean optics]]; that is, they used a [[convex lens|convex]] [[objective (optics)|objective]] and a [[concave lens|concave]] [[eyepiece|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 [[loupe]]s 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.<ref>[http://books.google.com/books?id=ngGzZe-5PBYC&pg=PA65&dq=Galilean+loupe+binoculars&hl=en&sa=X&ei=Er5tT5zBOIns0gGCnenBBg&ved=0CFQQ6AEwAQ#v=onepage&q=Galilean%20loupe%20binoculars&f=false Mark E. Wilkinson Essential Optics Review for the Boards, page 65]</ref> These are typically mounted on an eye-glass frame or custom-fit onto eyeglasses.


=== Binoculars with Keplerian optics ===
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An improved image and higher magnification is achieved in binoculars employing [[Keplerian Telescope|Keplerian optics]], where the image formed by the objective lens is viewed through a positive eyepiece lens (ocular).
'''source'''
Since the Keplerian configuration produces an inverted image, different methods were used to turn the image right way up.
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==== Binoculars with erecting lenses ====
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[[Image:Imageinverting-2.png|thumb|left|Cross-section of relay lens assembly - System 2.]]
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.<ref>http://fp.optics.arizona.edu/antiques/History%20of%20Telescopes%20and%20Binoculars%20-%20SPIE.pdf John E. Greivenkamp∗ and David L. Steed. The History of Telescopes and Binoculars:
An Engineering Perspective. Novel Optical Systems Design and Optimization XIV, edited by R. John Koshel, G. Groot Gregory, Proc. of SPIE Vol. 8129, 81290S-1 © 2011 SPIE CCC code: 0277-786X/11/$18 · doi: 10.1117/12.904614</ref>


==== Prism binoculars ====
==Demos==
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.<ref>Michael D. Reynolds, Mike D. Reynolds, Binocular Stargazing, Stackpole Books - 2005, page 8</ref>


====Porro prism binoculars====
Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]:
[[File:Double-porro-prism.png|thumb|left|Double Porro prism design]]
[[File:Koogan binoculars 01.JPG|thumb|right|Porro prism binoculars]]
''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 AG|Carl Zeiss company]] in the 1890s.<ref name="europa" /> 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 [[eyepiece]]s. 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.


====Roof-prisms binoculars====
[[File:Abbe-koenig-prism.png|thumb|left|Abbe-Koenig "roof" prism design]]
[[File:Leica Trinovid 8x20 BC.jpg|thumb|right|Binoculars with Schmidt-Pechan "roof" prisms]]
Binoculars using [[roof prism]]s may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse.<ref name="google">{{cite web|url=http://groups.google.co.ke/group/sci.astro.amateur/tree/browse_frm/month/2002-08/5a0a50e6887feb69?rnum=71&_done=%2Fgroup%2Fsci.astro.amateur%2Fbrowse_frm%2Fmonth%2F2002-08%3F |title=groups.google.co.ke |publisher=groups.google.co.ke |date= |accessdate=2009-11-03}}</ref><ref name="PhotoDigital">[http://www.photodigital.net/lists/rec.photo.equipment.misc/4/0455.html photodigital.net]&nbsp;— rec.photo.equipment.misc Discussion: Achille Victor Emile Daubresse, forgotten prism inventor</ref> 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|Porro-prism]] binoculars will inherently produce a brighter image than [[roof prism|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#Collimation and decollimation|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.<ref>{{cite book|url=http://books.google.com/books?id=piwP9HXtpvUC&pg=PA34&lpg=PA34&dq=%22porro+prism%22+binoculars+produce+brighter+image+than+%22roof+prism%22#PPA34,M1 |title='&#39;'Astronomy Hacks'&#39;' By Robert Bruce Thompson, Barbara Fritchman Thompson, chapter 1, page 34 |publisher=|date= 2005-06-24|accessdate=2009-11-03|isbn=9780596100605|author1=Thompson|first1=Robert Bruce|last2=Thompson|first2=Barbara Fritchman}}</ref>
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=== Optical parameters ===
==Test pages ==


[[File:Binoculars description plate2.jpg|thumb|Parameters listed on the prism cover plate describing 7 power [[magnification]] binoculars with a 50&nbsp;mm [[Objective (optics)|objective]] [[diameter]] and a 372-foot [[field of view]] at 1000 yards]]
To test the '''MathML''', '''PNG''', and '''source''' rendering modes, please go to one of the following test pages:
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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:
*[[Inputtypes|Inputtypes (private Wikis only)]]
 
*[[Url2Image|Url2Image (private Wikis only)]]
* Magnification: 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 7 times larger than the original seen from that distance. The amount of magnification depends upon the application the binoculars are designed for. Hand-held binoculars have lower magnifications so they will be less susceptible to shaking. A larger magnification leads to a smaller field of view.
==Bug reporting==
* Objective diameter: The [[diameter]] of the [[objective (optics)|objective lens]] determines how much light can be gathered to form an image. This number directly affects performance. When magnification and quality is equal, the larger the second binocular number, the brighter the image as well as the sharper the image. An 8×40, then, will produce a brighter and sharper image than an 8×25, even though both enlarge the image an identical eight times. The larger front lenses in the 8×40 also produce wider beams of light (exit pupil) that leave the eyepieces. This makes it more comfortable to view with an 8×40 than an 8×25. It is usually expressed in millimeters. It is customary to categorize binoculars by the magnification × the objective diameter; e.g. ''7×50''.
If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de .
* Field of view: The [[field of view]] of a pair of binoculars is determined by its optical design. 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 [[angle|angular]] value of how many degrees can be viewed.
* Exit pupil: Binoculars 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 (anatomy)|iris]] of the human eye— about 7&nbsp;mm, reducing with age. If the cone of light streaming out of the binoculars is ''larger'' than the pupil it is going into, any light larger than the pupil is wasted and does not provide information to the eye. In daytime use the human pupil is typically dilated about 3&nbsp;mm, which is about the exit pupil of a 7×21 binocular. Much larger 7×50 binoculars will produce a cone of light bigger than the pupil it is entering, and this light will, in the day, be wasted. It is therefore seemingly pointless to carry around a larger instrument. However, a larger exit pupil makes it easier to put the eye where it can receive the light: anywhere in the large exit pupil cone of light will do. This ease of placement helps avoid [[vignetting]], which is a darkened or obscured view that occurs when the light path is partially blocked. And, it means that the image can be quickly found which is important when looking at birds or game animals that move rapidly, or by a seaman on the deck of a pitching boat or ship. Narrow exit pupil binoculars may also be fatiguing because the instrument must be held exactly in place in front of the eyes to provide a useful image. Finally, many people use their binoculars at dusk, in overcast conditions, and at night, when their pupils are larger. Thus the daytime exit pupil is not a universally desirable standard. For comfort, ease of use, and flexibility in applications, larger binoculars with larger exit pupils are satisfying choices even if their capability is not fully used by day.
* Eye relief: [[Eye relief]] is the distance from the rear eyepiece lens to the exit pupil or eye point.<ref>"Introduction to Optics 2nd ed"., pp.141-142, Pedrotti & Pedrotti, Prentice-Hall 1993</ref> It is the distance the observer must position his or her eye behind the eyepiece in order to see an unvignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Binoculars may have eye relief ranging from a few millimeters to 2.5 centimeters or more. Eye relief can be particularly important for eyeglass wearers. The eye of an eyeglass wearer is typically further from the eye piece which necessitates a longer eye relief in order to still see 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.
* Close focus distance: Close focus distance is the closest point that the binocular can focus on. This distance varies from about 0.5m to 30m, depending upon the design of the binoculars.
 
== Mechanical design ==
 
=== Focus and adjustment ===
[[File:Fernglas.jpg|thumb|200px|Central-focusing binoculars with adjustable interpupillary distance]]
Binoculars have a [[focus (optics)|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":
 
*''Independent focus'' is an arrangement where the two telescopes are focused independently by adjusting each eyepiece. Binoculars designed for heavy field use, such as military applications, traditionally have used independent focusing.
 
*''Central focusing'' is an arrangement which involves rotation of a central focusing wheel to adjust both tubes together. 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). Because the focal change effected by the adjustable eyepiece can be measured in the customary unit of refractive power, the ''diopter'', the adjustable eyepiece itself is often called 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.
 
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.<ref>
{{cite web
| title = Self Focusing Binoculars (Fixed Focus): Always in Focus Binoculars
| url = http://www.bestbinocularsreviews.com/self_focusing_binoculars.php
| work = Best Binoculars & Binocular Reviews Website
| accessdate = 16 June 2012
}}</ref>
 
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.{{Citation needed|date=January 2012}} People with severe astigmatism, however, may still need to use their glasses while using binoculars.
[[File:PeopleBirding.JPG|thumb|People 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 lens|zoom camera lens]]. These designs are noted to be a compromise and even a [[gimmick]]<ref>[http://books.google.com/books?id=WfxnqueHQmEC&pg=PA54&dq=zoom+binoculars+compromise&lr=&cd=1#v=onepage&q=zoom%20binoculars%20compromise&f=false Pete Dunne, Pete Dunne on bird watching: the how-to, where-to, and when-to of birding, page 54]</ref> 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.<ref>[http://books.google.com/books?id=2lIwU313wgkC&pg=PT65&dq=zoom+binoculars&lr=&cd=14#v=onepage&q=zoom%20binoculars&f=false Philip S. Harrington, Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using, page 54]</ref> Models also have to match the magnification for both eyes throughout the zoom range and hold collimation to avoid eye strain and fatigue.<ref>[http://books.google.com/books?id=ac6wseOonlcC&pg=PT9&dq=zoom+binoculars+resolution&lr=&cd=11#v=onepage&q=zoom%20binoculars&f=false Stephen F. Tonkin, Binocular astronomy, page 46]</ref>
 
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.<ref name="thebinocularsite">[http://www.thebinocularsite.com/consumer/binoculars-for-children.html thebinocularsite.com]&nbsp;—A Parent's Guide to Choosing Binoculars for Children</ref>
 
=== Image stability ===
 
Some binoculars use [[image stabilization|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.
 
=== Alignment ===
 
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.<ref>Stephen Mensing, Star gazing through binoculars: a complete guide to binocular astronomy, page 32</ref>
 
Alignment is performed by small movements to the prisms, by adjusting an internal support cell or by turning external [[set screw]]s, or by adjusting the position of the objective via [[eccentric (mechanism)|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.
 
== Optical coatings ==
{{Main|Optical coating}}
[[File:DFRBinoculars.jpg|thumb|Binoculars with red-colored multicoatings]]
Since a typical binocular has 6 to 10 optical elements <ref>[http://books.google.com/books?id=piwP9HXtpvUC&pg=PA35&dq=binocular+optical+coatings&lr=&cd=13#v=onepage&q=binocular%20optical%20coatings&f=false Robert Bruce Thompson, Barbara Fritchman Thompson, Astronomy hacks, page 35]</ref> with special characteristics and up to 16 air-to-glass surfaces, binocular manufactures use different types of [[optical coating]]s for technical reasons and to improve the image they produce.
 
===Anti-reflective coatings===
{{Main|Anti-reflective coating}}
[[Anti-reflective coating]]s reduce light lost at every optical surface through [[reflection (physics)|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.
 
=== Phase correction coatings ===
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 [[Polarization (waves)|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 (wave propagation)|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.<ref>http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/09/189.aspx</ref> These unwanted interference effects can be suppressed by [[Chemical vapor deposition|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 prism|Schmidt-Pechan roof prism]] or an [[Abbe-Koenig prism|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.
 
=== Metallic mirror coatings ===
{{Main|Mirror}}
In binoculars with [[Schmidt-Pechan prism|Schmidt-Pechan roof prism]]s, 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 (optics)|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.<ref>{{cite web|url=http://www.zbirding.info/Truth/prisms/prisms.htm |title=www.zbirding.info |publisher=www.zbirding.info |date= |accessdate=2009-11-03}}</ref>
 
[[Porro prism]] binoculars and roof prism binoculars using the [[Abbe-Koenig prism|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 mirror coatings ===
{{Main|Dielectric mirror}}
Dielectric coatings are used in [[Schmidt-Pechan prism|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 [[Electromagnetic spectrum#Visible radiation .28light.29|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 prism|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.
 
=== Terms used to describe coatings ===
[[File:Navy binoculars.jpg|thumb|right|Special reflective coatings on large naval binoculars]]
 
==== For all binoculars ====
The presence of any coatings is typically denoted on binoculars by the following terms:
 
* ''coated optics'': one or more surfaces are anti-reflective coated with a single-layer coating.
* ''fully coated'': all air-to-glass surfaces are anti-reflective coated with a single-layer coating. Plastic lenses, however, if used, may not be coated{{Citation needed|date=August 2008}}.
* ''multi-coated'': one or more surfaces have anti-reflective multi-layer coatings.
* ''fully multi-coated'': all air-to-glass surfaces are anti-reflective multi-layer coated.
 
==== For binoculars with roof prisms only (not needed for Porro prisms) ====
* ''phase-coated'' or ''P-coating'': the roof prism has a phase-correcting coating
* ''aluminium-coated'': the roof prism mirrors are coated with an aluminium coating. The default if a mirror coating isn't mentioned.
* ''silver-coated'': the roof prism mirrors are coated with a silver coating
* ''dielectric-coated'': the roof prism mirrors are coated with a dielectric coating
 
== Applications ==
 
=== General use ===
[[File:Tower Optical Binoculars.jpg|thumb|[[Tower Optical]] coin-operated binoculars]]
Hand-held binoculars range from small 3&nbsp;×&nbsp;10 Galilean [[opera glasses]], used in [[theater]]s, to glasses with 7 to 12 diameters magnification and 30 to 50&nbsp;mm objectives for typical outdoor use.
 
Many [[tourist attraction]]s have installed pedestal-mounted, coin-operated binoculars to allow visitors to obtain a closer view of the attraction.
 
=== Range finding ===
 
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&nbsp;[[angular mil|mil]].<ref name="bushnell">[http://www.binoculars.com/images/pdf/BUP336.pdf Binoculars.com]&nbsp;— Marine 7 × 50 Binoculars. Bushnell</ref> 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:
 
:<math>D = \frac{OH}{\text{Mil}}\times 1000</math>
where:
 
* <math>D</math> is the ''Distance'' to the object in meters.
* <math>OH</math> is the known ''Object Height''.
* <math>\text{Mil}</math> is the angular height of the object in number of ''Mil''.
 
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.
 
:<math>8000 \text{m} = \frac{120 \text{m}}{15 \text{mil}} \times 1000</math>
 
=== Military ===
[[File:US Navy 021206-N-1328C-501 Signalman 3rd Class Tiffany Culereth from Bronx, N.Y., observes ships in the area through binoculars called ^ldquo,Big Eyes.^rdquo,.jpg|thumb|Naval ship binoculars]]
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 [[Infrared|IR emissions]], while modern ones usually are fitted with filters blocking [[Laser beam#As weapons|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 [[rangefinder]]s (up to 15 meters separation of the two objective lenses, weight 10 tons, for ranging [[World War II]] naval gun targets 25&nbsp;km away) have been used, although late-20th century technology made this application redundant.
 
=== Astronomical ===
[[File:25x150binocular.jpg|thumb|25&nbsp;×&nbsp;150 binoculars adapted for astronomical use]]
 
Binoculars are widely used by [[amateur astronomy|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&nbsp;mm or 80&nbsp;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&nbsp;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. A magnification of 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 making|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&nbsp;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&nbsp;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&nbsp;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.<ref name=ST2012>[[Sky & Telescope]], October 2012, Gary Seronik, "The Messier Catalog: A Binocular Odyssey" (pg 68)</ref>
[[File:Galassia di Andromeda tel114.png|thumb|A simulated view of how the [[Andromeda galaxy]] (Messier 31) would appear in a pair of binoculars]]
Some [[open clusters]], such as the bright double cluster ([[NGC 869]] and [[NGC 884]]) in the constellation [[Perseus]], and [[globular clusters]], such as [[Messier 13|M13]] in Hercules, are easy to spot. Among nebulae, [[Messier 17|M17]] in [[Sagittarius (constellation)|Sagittarius]] and the [[North American nebula]] ([[NGC 7000]]) in Cygnus are also readily viewed. Binoculars can show a few of the wider-split [[Binary stars|binary star]] such as [[Albireo]] in the constellation [[Cygnus (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 (dwarf planet)|Ceres]], [[Vesta (asteroid)|Vesta]] and [[Pallas (asteroid)|Pallas]]; Saturn's largest moon [[Titan (moon)|Titan]]; and the [[Galilean moons]] of [[Jupiter]]. Although visible unaided in [[air pollution|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.<ref name="binoculars">{{cite web
  |year=2004
  |title=Limiting Magnitude in Binoculars
  |publisher=Cloudy Nights
  |author=Ed Zarenski
  |url=http://www.cloudynights.com/documents/limiting.pdf
  |accessdate=2011-05-06}}</ref> Asteroids like [[704 Interamnia|Interamnia]], [[511 Davida|Davida]], [[52 Europa|Europa]] and, unless under exceptional conditions [[10 Hygiea|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 planet]]s [[Pluto]] and [[Eris (dwarf planet)|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.
 
== List of binocular manufacturers ==
{{Disputed-section|date=September 2010}}
<!--NOTE: This is a list of manufacturers that have Wikipedia articles and are noted in that article text or some other source as being a "manufacturer". Not all companies that sell binoculars manufacture them, see http://www.cloudynights.com/ubbthreads/attachments/993551-JpnSurvy.txt for a potential source-->
There are many companies that manufacturer binoculars, both past and present. They include:
* [[Barr and Stroud]] (UK)&nbsp;— sold binoculars commercially and primary supplier to the Royal Navy in [[World War II|WWII]]. The new range of Barr & Stroud binoculars are currently made in China (Nov. 2011) and distributed by Optical Vision Ltd.
* [[Bausch & Lomb]] (USA)&nbsp;— has not made binoculars since 1976, when they licensed their name to Bushnell, Inc., who made binoculars under the Bausch & Lomb name until the license expired, and was not renewed, in 2005.
* [[Bushnell Corporation]] (USA).
* Bosma (Guangzhou Bosma Corp) (China). A leading manufacturer of sports optics such as binoculars, rifle scopes, dot sights, spotting scopes, etc. Bosma Optics<ref>[http://www.bosmaoptics.com/]</ref>
* [[Canon (company)|Canon Inc]] (Japan)&nbsp;— I.S. series: porro variants?
* [[Celestron]].
* [[DOCTER (optics)]] (Germany) - Nobilem series (Porro prisms).
* [[Fujinon]] (Japan)&nbsp;— FMTSX, FMTSX-2, MTSX series: porro.
* J.O.C. Guangzhou Jinghua Optics and Electronic Co., LTD (China) - Large original equipment manufacturer and part owner of Bresser (DE and USA), Meade and Explore Scientific.<ref>[http://bresser.en.alibaba.com/company_profile.html bresser.en.alibaba.com - Guangzhou Jinghua Optics & Electronics Co., Ltd.]</ref>
* [[I.O.R.]] (Romania).
* Kamakura Koki Co., Ltd. - Large [[original equipment manufacturer]] manufacturer with factories in Japan and in China for companies such as Bushnell, Alpen, Zen Ray, Eagle Optics, Leupold & Stevens, Vixen.<ref>[http://panjiva.com/Kamakura-Koki-Co-Ltd/1523748 panjiva.com - Limited Company Profile Kamakura Koki Co., Ltd. Supplier — Japan]</ref>
* [[Konus]] (Italy).
* [[Leica Camera]] (Germany)&nbsp;— Ultravid, Duovid, Geovid, Trinovid: all are roof prism.
* [[Leupold & Stevens|Leupold & Stevens, Inc]] (USA).
* [[Meade Instruments]] (USA)– Glacier (roof prism), TravelView (porro), CaptureView (folding roof prism) and Astro Series (roof prism). Also sells under the name ''Coronado''.
* [[Meopta]] (Czech Republic)&nbsp;— Meostar B1 (roof prism).
* [[Minox]].
* Miyauchi (Japan).
* [[Nikon]] (Japan)&nbsp;— EDG Series, High Grade series, Monarch 3, 5, 7 series, RAII, Spotter series: roof prism; Prostar series, Superior E series, E series, Action EX series: porro. Prostaff series, Aculon series.
* [[Olympus Corporation]] (Japan).
* [[Pentax]] (Japan)&nbsp;— DCFED/SP/XP series: roof prism; UCF series: inverted porro; PCFV/WP/XCF series: porro.
* [[Steiner Optics|Steiner]] (Germany).<ref>{{cite web|url=http://www.steiner-binoculars.com|title=www.steiner-binoculars.com|date= |accessdate=2009-12-21}}</ref>
* [[Sunagor]] (Japan).
* [[Swarovski Optik]].<ref>{{cite web|url=http://www.regionhall.at/en/the-swarovski-story.html |title=www.regionhall.at&nbsp;—The Swarovski story |publisher=Regionhall.at |date= |accessdate=2009-11-03}}</ref>
* [[Takahashi Seisakusho]] (Japan).
* [[Vixen (telescopes)]] (Japan)&nbsp;— Apex/Apex Pro: roof prism; Ultima: porro.
* [[Vivitar]] (USA).
* [[Vortex Optics]] (USA).
* [[Yukon Optics]] (Worldwide).
* Yunnan Optoelectronics Co. Ltd., based in Kunming, manufactures binoculars for Oberwerk, Ohio USA in their joint venture YunAo Optics Co. Ltd.<ref>[http://www.thebinocularsite.com/oberwerk/ The Binocular Site - Oberwerk Binoculars]</ref>
* [[Carl Zeiss AG|Zeiss]] (Germany)&nbsp;— FL, Victory, Conquest: roof prism; 7×50 BGAT/T porro, 15×60 BGA/T porro, discontinued.
 
== See also ==
* [[Anti-fog]]
* [[Binoviewer]]
* [[Globe effect]]
* [[Lens (optics)|Lens]]
* [[List of telescope types]]
* [[Monocular]]
* [[Optical telescope]]
* [[Spotting scope]]
* [[Tower viewer]]
 
== References ==
{{Reflist|2}}
 
==Further reading==
*{{Cite EB1911 |wstitle=Binocular Instrument |volume=3 |pages=949-951 }}<!-- When it goes into Wikisource, replace the above with an {{EB1911 poster|}} entry. -->
* Walter J. Schwab, Wolf Wehran: "Optics for Hunting and Natur Observation". ISBN 978-3-00-034895-2. 1st Edition, Wetzlar (Germany), 2011
 
== External links ==
{{Commons category}}
 
<!-- ==============================({{NoMoreLinks}})============================== -->
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<!-- ============================================================================= -->
* [http://www.nightskyinfo.com/binoculars A Guide to Binoculars] by Emil Neata
* [http://www.europa.com/~telscope/binotele.htm The history of the telescope & the binocular] by Peter Abrahams, May 2002
 
[[Category:Optical devices]]
 
[[oc:Jumelles]]

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