binoculars

binoculars

[buh-nok-yuh-ler, bahy-]

Optical instrument for providing a magnified view of distant objects, consisting of two similar telescopes, one for each eye, mounted on a single frame. In most binoculars, each telescope has two prisms, which reinvert the inverted image provided by the eyepiece of each telescope. Light rays travel along a folded path inside the telescopes, so the instrument has a shorter overall length. The prisms also provide better depth perception at greater distances, by allowing the two objectives (object lenses) to be set farther apart than the eyepieces. Binocular eyepieces are often fitted to microscopes or other optical instruments.

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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.

Optical design

Galilean binoculars

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.

Prism binoculars

An improved image and higher magnification can be achieved in a construction binoculars employing Keplerian optics, where the image formed by the objective lens is viewed through a positive eyepiece lens (ocular). This configuration has the disadvantage that the image is inverted. There are different ways of correcting these disadvantages.

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 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 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.

Relative advantages of porro prism and roof prism binoculars

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.

Optical parameters

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.

Optical coatings

Anti-reflective coatings

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.

Roof prism phase correction coating

In binoculars with roof prisms multiple internal reflections in a roof prism cause a polarization-dependent phase-lag of the transmitted light, in a manner similar to a Fresnel rhomb.

The light path through the roof prism is split in two paths that reflect on either side of the roof 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. During any reflection, including total internal reflection inside a prism, unpolarized light becomes partially polarized. 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 vector called the phase shift, or the geometric phase, or the Berry phase.

In a roof prism without a phase correcting coating interference between the two paths with different geometric phase results in an varying intensity distribution in the image reducing apparent contrast and resolution compared to a porro prism erecting system. This effect can be seen in the elongation of the Airy disk in the same direction as the crest of the roof.

The unwanted interference effects are 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 a 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.

Roof prism metallic mirror coating

In binoculars that use a Schmidt-Pechan roof prism some surfaces of the roof prism must be mirror coated for efficient reflection since the light is incident at one of the glass-air boundaries at an angle less than the critical angle so total internal reflection does not take occur. Without a mirror coating most of that light would be lost. Typically an aluminum mirror coating (reflectivity of 87% to 93%) or silver mirror coating (reflectivity of 95% to 98%) is used.

In older binocular designs silver mirror coatings were used but these coatings oxidized and lost reflectivity over time in unsealed binoculars. Aluminum mirror coatings were used in later unsealed designs because it did not tarnish even though it has a lower reflectivity than silver. Modern binocular designs use either aluminum or silver. Silver is used in modern high-quality designs as modern binoculars are sealed and nitrogen or argon filled so the silver mirror coating doesn't tarnish in an inert atmosphere.

Porro prism binoculars and roof prism binoculars using the Abbe-Koenig roof prism do not use mirror coatings because these prisms reflect with 100% reflectivity using total internal reflection in the prism.

Roof prism dielectric mirror coating

A dielectric coating on a Schmidt-Pechan roof prism causes 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 aluminum 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.

Marketing terms used to denote coatings

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.
  • 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.
  • phase-coated or P-coating: the roof prism has a phase-correcting coating
  • aluminum-coated: the roof prism mirrors are coated with a aluminum 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

Mechanical design

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). 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 idiotically 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 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.

Image stabilization

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 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

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.

Applications

General use

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.

Military

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 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. 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.

Astronomical

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). Ceres, Neptune, Pallas, Titan, and the Galilean moons of Jupiter are invisible to the naked eye but can readily be seen with binoculars. Although 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.

Manufacturers

Some notable binocular manufacturers as of 2008. Sorted in alphabetical order:

See also

Notes

References

External links

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