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. Most are sized to be held using both hands, although there are much larger types.
Unlike a monocular telescope, a binocular gives 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.
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 theatre 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 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 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 have objective lenses that are approximately in line with the eyepieces.
Porro vs. Roof prisms
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 as far as 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 won't 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:
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 as if one were 7 times closer to the object. 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.
Objective diameter – The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
It is customary to categorize binoculars by the magnification × the objective diameter; e.g. 7×50.
Field of view — The field of view of a binocular 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 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 of the human eye— about 7 mm, reducing with age. Light gathered 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 larger exit pupil makes alignment of the eye easier and avoids dark vignetting intruding from the edges.
Eye relief — 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 see an unvignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Binoculars 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 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.
Since a binocular can have 16 air-to-glass surfaces, with light lost at every surface, optical coatings can significantly affect 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 eliminated, 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 binocular, 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 binoculars 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.
A classic lens-coating material is magnesium fluoride; it reduces reflections 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. For roof-prisms, anti-phase shifting coatings are sometimes used which significantly improve contrast. The presence of a coating is typically 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.
Focusing and adjustment
Binoculars 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. Binoculars with "independent focus" require the two telescopes to be focused independently by adjusting each eyepiece, thereby changing the distance between ocular and objective lenses. Binoculars designed for heavy field use, such as military applications, traditionally have used independent focusing. Because general users find it more convenient to focus both tubes with one adjustment 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.
There are also "focus-free" or "fixed-focus" binoculars. 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 binoculars, while in principle a good idea, are generally considered not to perform very well.
Most modern binoculars 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.
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 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.
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 close to circular anyway. The cinematic convention used to represent a view through binoculars as two circles partially overlapping in a figure-of-eight shape is not true to life.
Misalignment is remedied by small movements to the prisms, often by turning screws accessible without opening the binoculars, or by adjusting the position of the objective via eccentric rings built into the objective cell. Alignment is usually done by a professional although instructions for checking binoculars for collimation errors and for collimating them can be found on the Internet.
Hand-held binoculars range from small 3 x 10 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 binoculars to allow visitors to obtain a closer view of the attraction. In the United Kingdom, 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.
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 heavily ruggedized than their civilian counterparts. They generally avoid more fragile center focus arrangements in favour of independent focus. 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. 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, 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 binocular is 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 making them useful for comet and supernova seeking (giant binoculars) and general observation (portable binoculars). 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 binoculars for practical observation. 10×50 binoculars 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 binoculars. Likewise too faint to be seen with binoculars are all moons except the Galileans and Titan, and the dwarf planets Pluto and Eris.
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 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 binoculars usually require some sort of mount
Much larger 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 binoculars, is the Large Binocular 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 allowing interferometric use.
Some notable binocular manufacturers as of 2008:
- Leica GmbH – Ultravid, Duovid, Geovid: all are roof prism.
- Optolyth – Royal, ViaNova: roof prism; Alpin, Alpin Classic: porro prism.
- Zeiss GmbH – FL, Victory, Conquest: roof prism; 7×50 BGAT/T porro, 15×60 BGA/T porro, discontinued.
- Eschenbach Optik GmbH – Farlux, Trophy, Adventure, Sektor...: some are roof prism, some porro.
- Docter (the former Carl Zeiss Jena plant in Eisfeld) Nobilem 7×50, 8×56, 10×50, 15×60: porro; Docter 7×40, 8×40, 10×40: roof prism.
- Steiner GmbH – Commander, Nighthunter: porro; Predator, Wildlife: roof prism.
- Delta Optical – binoculars, riflescopes, microscopes
- Swarovski Optik – SLC, EL: roof prism; Habicht: porro prism, but to be discontinued.
- Optolyth – Royal: Roof; Alpin: porro
- KAHLES – riflescopes, binoculars
- Canon Inc. – I.S. series: porro variants?
- Nikon Co. – High Grade series, Monarch series, RAII, Spotter series: roof prism; Prostar series, Superior E series, E series, Action EX series: porro.
- Fujinon Co. – FMTSX, FMTSX-2, MTSX series: porro.
- Kowa Co. – BD series: Roof prism.
- Pentax Co. – DCFSP/XP series: roof prism; UCF series: inverted porro; PCFV/WP/XCF series: porro.
- Olympus Co. – EXWPI series: roof prism.
- Vixen Co. – Apex/Apex Pro: roof prism; Ultima: porro*
- Miyauchi Co. – specializes in oversized porro binoculars.
* Also sells OEM products manufactured by the Kamakura Koki Co. Ltd. of Japan.
In the early 21st century, some mid-priced binoculars have become available in the internal Chinese market. A few are said to be comparable in both performance and price to those of some of the better brands, but the great majority are inferior.
- Sicong (from Xian Stateoptics) – Navigator series: roof prism; Ares series: porro.
- WDtian (from Yunnan State optics) – porro.
- Yunnan State optics – MS series: porro.
- Brunton, Inc.
- Bushnell Performance Optics*
- Carson Optical
- Leupold & Stevens, Inc.*
- Vortex Optics
- William Optics
- Zen-Ray Optics – SUMMIT, Vista Series WP.
* Also sells OEM products manufactured by the KAMAKURA KOKI CO. LTD. of Japan.
- Yukon Advanced Optics
- Russian Military Binoculars – BPOc 10x42 7x30, BKFC series.