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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.