2007 Schools Wikipedia Selection. Related subjects: Engineering

Porro-prism binoculars with central focusing
Porro-prism binoculars with central focusing

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 adequate to be held using both hands although there are much larger types.

Unlike a monocular telescope, binoculars give users a three-dimensional image: the two views, presented from slightly different viewpoints to each of the viewer's eyes, merge to produce a single perceived view with a sensation of depth, allowing distances to be estimated. Binoculars are also more comfortable for viewing, as they negate the need to close or obstruct one eye to avoid confusion. It is also easier and more comfortable to steadily hold and move a pair of binoculars than a single tube, as the two hands and the head form a steady three-point platform.

A typical Porro prism binocular design
A typical Porro prism binocular design

Optical design

Galilean binoculars
Galilean binoculars

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 construction is still used in very cheap models and in " opera glasses".

Porro prism binoculars

Double Porro prism design
Double Porro prism design

Named after Italian optician Ignazio Porro who patented this image erecting system in 1854 and later refined by makers like Carl Zeiss in the 1890's , 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

Abbe-Koenig "roof prism" design
Abbe-Koenig "roof prism" design

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 are narrower, more compact, and more expensive than those that use Porro prisms. They have objective lenses that are approximately in line with the eyepieces.

Porro vs. Roof prisms

Aside from the difference in price and portability noted above these two designs have effects on reflections and brightness. Porro-prism binoculars will inherently produce an intrinsically brighter image than roof-prism binoculars of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path. However, as of 2005, the optical quality of the best roof-prism binoculars with up-to-date coating processes as used in Schmidt-Pechan models is comparable with the best Porro glasses, and it appears that roof prisms will dominate the market for high-quality portable binoculars in spite of their higher price. The major European optical manufacturers (Leica, Zeiss, Swarovski) have discontinued their Porro lines; Japanese manufacturers (Nikon, Fujinon, etc.) may follow suit.

Optical parameters

Binoculars are usually rated by objective diameter, magnifying power, and field of view. The diameter of the objective lenses determines the light-gathering power and the theoretical resolving power of the binoculars. The ratio of the focal lengths of the objective and the eyepiece gives the linear magnifying power (expressed in "diameters"). It is customary to categorise binoculars by the magnification × the objective diameter in mm; e.g. 7×50. A magnification of factor 7, for example, produces an image as if one were 7 times closer to the object. The resulting “field of view” is measured in the width viewed at factor 1000 (in meters or feet).

The magnification required depends upon the application, but with the major proviso that large magnifications give an image much more susceptible to shake when hand-held. A larger the magnification also leads to a smaller the field of view. Binoculars with lower magnification may well show more detail because they can be held steadily and the larger fields of view can contribute to a more natural viewing experience. For general hand-held use, subject to shake, 7 to 8 diameters is a good compromise between power and image steadiness for most people. Binoculars of 7×30 or 8×30 power are good for daytime use. For general night use, a 50 mm objective gives maximum brightness; objective diameter must be increased for higher magnifications at night.

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 human eye—about 7 mm, reducing with age. Light gathered by a larger exit pupil is wasted. The current trend favors models with 5 mm exit pupil, such as 10x50 or 8x40, while 7x50 is falling out of favour. 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 to intrude from the edges.

Optical coatings

U.S. Navy binoculars
U.S. Navy binoculars

Since binoculars may have 16 air-to-glass surfaces, with light lost at every surface, optical coatings can significantly affect their image quality. When light strikes an interface between two materials of different refractive index (e.g., at an air-glass interface), some of the light is transmitted, some reflected. In any sort of image-forming optical instrument (telescope, camera, microscope, etc.), ideally no light should be reflected; instead of forming an image, light which reaches the viewer after being reflected is distributed in the field of view, and reduces the contrast between the true image and the background. Reflection can be reduced, but not 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 binoculars, 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.

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 a pair of 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.

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). 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 general considered not 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 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, and stabilized binoculars contain more advanced technology to go wrong, and to become obsolete. They are also more expensive, heavier, and battery life tends to be short. Stabilization is not suitable when tracking moving objects.


A well-collimated pair of 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.


Coin-operated binoculars
Coin-operated binoculars

General use

Hand-held binoculars range from small 3x10 Galilean opera glasses, used in theaters, to glasses with 7 to 12 diameters magnification and 30 to 50 mm objectives for typical outdoor use. Porro prism models predominate although bird watchers and hunters tend to prefer, and are prepared to pay for, the lighter but more expensive roof-prism models.

Many tourist attractions have installed pedestal-mounted, coin-operated 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.


Naval ship binoculars
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 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.

There are binoculars designed specifically for civilian and military use at sea. Hand held models will be 5x to 7x but with very large prism sets combined with eyepieces designed to give generous eye relief. This optical combination allows the user to see through them even when they are pitching and vibrating in relationship the viewers eye without the image vignetting or going dark. Large permanently mounted models with large objectives operating at higher magnification are also used.

Very large “binocular” naval rangefinders (up to 15 meters, 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; there wide field of view making them useful for comet and supernova seeking (giant binoculars) and general observation (portable binoculars).

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 marketed for specifically for astronomical use will tend to have higher magnification combined with a larger aperture objective because the diameter of the objective lens determines the faintest star you can see.

Much larger binoculars have been made by amateur telescope makers, essentially using two refracting or reflecting astronomical telescopes, with mixed results. A very large example in the professional astronomical world, 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 certainly not intended to be held to the eyes of a viewer, it uses two telescopes to view the same object, giving additional information due to the larger field of view that results from the separation of the objective mirrors.


Some notable binocular manufacturers as of 2005:

1. European brands

  • Leica GmbH (Ultravid, Duovid, Geovid: all are Roof)
  • Swarovski Optik (SLC, EL: all are Roof; Habicht: Porro, but to be discontinued)
  • Zeiss GmbH (FL,Victory, Conquest: all are Roof; 7x50 BGAT/T: Porro, 15x60 BGA/T Porro, discontinued)
  • Eschenbach Optik GmbH (Farlux, Trophy, Adventure, Sektor...; some are Roof, some are Porro)
  • Docter Optik (Nobilem: Porro)
  • Optolyth (Royal: Roof; Alpin: Porro)
  • Steiner GmbH (Commander, Nighthunter: Porro; Predator, Wildlife: Roof)
  • Russian Military Binoculars (BPOc 10x42 7x30, BKFC series)

2. Japanese brands

  • Canon Co. (I.S. series, Porro variants?)
  • Nikon Co. (High Grade series, Monarch series,RAII, Spotter series: Roof; Prostar series, Superior E series, E series, Action EX series: Porro)
  • Fujinon Co. (FMTSX, MTSX series: Porro)
  • Kowa Co. (BD series: Roof)
  • Pentax Co. (DCFSP/XP series; Roof, UCF series: Inverted Porro; PCFV/WP/XCF series: Porro)
  • Olympus Co. (EXWPI series: Roof)
  • Minolta Co. (Activa, some are Roof, some are Porro)
  • Vixen Co. (Apex/Apex Pro: Roof; Ultima: Porro)
  • Miyauchi Co. (Specialized in over-sized Porro binocualars)

P.S. Many of the above are OEM products of Kamakura or Chinese manufacturing plants.

3. Chinese brands

In the early years of the 21st century, some mid-priced binoculars have become available in the internal Chinese market. A few of them are said to be comparable both in performance and in price to some of the better brands, with the great majority of them being inferior.

  • Sicong (from Xian Stateoptics. Navigator series: Roof; Ares series: Porro)
  • WDtian (from Yunnan State optics, all Porro)
  • Yunnan State optics (MS series: Porro)

4. American brands

  • Bushnell Performance Optics
  • Leupold & Stevens, Inc.
  • Vortex Optics

5. Russian brands

  • Yukon Advanced Optics
  • Baigish
  • Kronos

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