Optical device
A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocular are typically less than half of a pair of binoculars with similar optical properties, making it more portable and also less expensive. This is because binoculars are essentially a pair of monoculars packed together — one for each eye. As a result, monoculars only produce two-dimensional images, while binoculars can use two parallaxed images (each for one eye) to produce binocular vision, which allows stereopsis and depth perception.
Monoculars are ideally suited to those applications where three-dimensional perception is not needed, or where compactness and low weight are important (e.g. hiking). Monoculars are also sometimes preferred where difficulties occur using both eyes through binoculars due to significant eyesight variation (e.g. strabismus, anisometropia or astigmatism) or unilateral visual impairment (due to amblyopia, cataract or corneal ulceration).
Conventional refracting telescopes that use relay lenses have a straight optical path that is relatively long; as a result, monoculars normally use Porro or roof prisms to "fold up" the optical path, which makes it much shorter and compact (see the entry on binoculars for details). However, monoculars also tend to have lower magnification factors than telescopes of the same objective size, and typically lack the capacity of variable magnification.
Visually impaired people may use monoculars to see objects at distances at which people with normal vision do not have difficulty, e.g., to read text on a chalkboard or projection screen. Applications for viewing more distant objects include natural history, hunting, marine and military. Compact monoculars are also used in art galleries and museums to obtain a closer view of exhibits.
When high magnification, a bright image, and good resolution of distant images are required, a relatively larger instrument is preferred (i.e. a telescope), often mounted on a tripod. A smaller pocket-sized "pocket scope" (i.e. a typical monocular) can be used for less stringent applications. These comments are quantified below.
Whereas there is a huge range of binoculars on the world market, monoculars are less widely available and with a limited choice in the top quality bracket, with some traditionally very high quality optical manufacturers not offering monoculars at all. Today, most monoculars are manufactured in Japan, China, Russia and Germany, with China offering more product variety than most. Prices range widely, from the highest specification designs listed at over £300 down to "budget" offerings at under £10. (As at February 2016).
Monocular sizes.
As with binoculars and telescopes, monoculars are primarily defined by two parameters: magnification and objective lens diameter, for example, 8×30 where 8 is the magnification and 30 is the objective lens diameter in mm (this is the lens furthest from the eye). An 8× magnification makes the distant object appear to be 8 times larger at the eye. Contemporary monoculars are typically compact and most normally within a range of 4× magnification to 10×, although specialized units outside these limits are available. Variable magnification or zoom is sometimes provided, but has drawbacks and is not normally found on the top quality monoculars. The objective lens diameter is typically in the range 20mm to 42mm. Care is needed in interpreting some monocular specifications where numerical values are applied loosely and inaccurately—e.g. "39×95", which on a small cheap monocular is more likely to refer to the physical dimensions than the optical parameters. (This is covered in more detail in the section "Interpreting product specifications" below.)
As with binoculars, possibly the most common and popular magnification for most purposes is 8×. This represents a usable magnification in many circumstances and is reasonably easy to hold steady without a tripod or monopod. At this magnification, the field of view is relatively wide, making it easier to locate and follow distant objects. For viewing at longer distances, 10× or 12× is preferable if the user is able to hold the monocular steady. However, increasing magnification will compromise the field of view and the relative brightness of the object. These and other considerations are major factors influencing the choice of magnification and objective lens diameter. Although very high numerical magnification sounds impressive on paper, in reality, for a pocket monocular it is rarely a good choice because of the very narrow field of view, poor image brightness, and great difficulty in keeping the image still when hand holding. Most serious users will eventually come to realize why 8× or 10× are so popular, as they represent possibly the best compromise and are the magnifications most commonly adopted in the very highest quality field monoculars (and binoculars).
Where a monocular end and a telescope start is debatable but a telescope is normally used for high magnifications ( ;20×) and with correspondingly larger objective lens diameter (e.g. 60–90mm). A telescope will be significantly heavier, more bulky, and much more expensive, than a monocular and, due to the high magnifications, will normally need a tripod, reflecting telescopes used for astronomy, typically, have inverted images. Most popular monocular sizes mimic popular binoculars – e.g. 7×25, 8×20, 8×30, 8×42, 10×42.
Design.
Much of the basic design considerations and related parameters are the same as for binoculars, and are covered in that entry, but some expanded comments have been added where appropriate:
Exit pupil is defined as the diameter of the objective lens divided by the magnification and expressed in mm. (e.g. an 8×40 will give an exit pupil diameter of 5mm). For a given situation, the greater the exit pupil, the better the light transmission into the eye. Hence, a large objective lens with a low magnification will give good light admission, which is especially important in deteriorating light conditions. The classic 7×50 marine binocular or monocular is ideally suited to low light conditions with its relatively large exit pupil diameter of 7.1mm and a realistic magnification which is practical on a moving boat. However, the exit pupil should be considered in relationship with the human eye pupil diameter. If the exit pupil of the chosen instrument is greater than the human eye pupil, then there will be no benefit, as the eye will be the limiting factor in light admission. In effect, the extra light-gathering potential is wasted. This is a consideration as one ages because human eye pupil dilation range diminishes with age, as shown as an approximate guide in the table below.
Field of view (FOV) and magnification are related; FOV increases with decreasing magnification and vice versa. This applies to monoculars, binoculars, and telescopes. However, this relationship also depends on optical design and manufacture, which can cause some variation. The following chart shows the FOV/magnification relationship based on best-in-class data, taken both from tests and manufacturers' specifications. Contrary to some belief, it is a myth that binoculars offer a wider field of view than monoculars. For a given specification and manufacturer offering, both monocular or binocular options of the same model, the field of view is exactly the same, whether monocular or binocular.
Eye relief is a particularly important (but often overlooked) parameter for spectacle wearers, if the full field of view is to be visible. Although magnification, objective lens diameter, and field of viewn(either in degrees or m [Tag]) are often shown on the body of the monocular, eye relief virtually never is (except, perhaps, to say "long eye relief" or "LER"). Early optics tended to have short eye relief, (sub-10mm) but more contemporary designs are much better. At least 15mm is desirable—ideally near 20mm—for spectacle wearers. (See table of eye reliefs below noting the best in class, Opticron 5×30 at 25mm and Opticron 8×42 DBA, at 21mm). Eye relief can seriously compromise the field of view if too short, so even if an optic has a good field of view specification, without an accompanying long eye relief, the benefit of the wide view will not be obtained (again, only applying to spectacle wearers). The eye lens diameter can greatly facilitate good eye relief. The photograph below shows a comparison between two 8× monoculars. The one on the left, typical of a 1980s-design, features a relatively small eyepiece lens diameter (11mm) and eye relief ( ;10mm). The one on the right is from 2016, featuring a comparatively larger eyepiece diameter (24mm) and eye relief (~15mm). This large eyepiece lens not only helps eye relief, but also helps to create a wider field of view. 
Two additional aspects which are particularly relevant in the context of monoculars are the following:
A significant difference between binoculars and monoculars is in the focusing system. Today, binoculars almost universally use a central wheel focusing system, operating on both sides simultaneously. Some large observation binoculars, as well as some older designs, feature individual focusing on each eyepiece. Monoculars, however, employ a variety of different focusing systems, all with pros and cons. These include the following:
The most common type is the focusing ring around the body. This retains the compactness of the unit, but requires two hands to operate and does not give particularly fast focusing. In some units, the ring can be stiff to operate.
The small ring near the eyepiece also usually needs two hands to operate, and, in some designs, can interfere with the twist-up eye cup. Being small, it can, also be less convenient to operate, especially whilst wearing gloves. The degree of twist, from closest focus to infinity, varies between manufacturers. Some use a very small twist (about a quarter of a turn), whereas others use a full turn or more. The small degree of twist gives a very fast focus, but can be overly sensitive, and, in some designs, be too stiff to use with one hand. A full turn is a practical compromise.
A focusing wheel tends not to be used on top quality monoculars (with the exception of the Bushnell 10×42HD Legend), but is particularly popular on budget offerings from China. Although it makes the monocular more bulky, it does give very convenient focusing with one hand (via one finger) and is particularly fast and smooth, which is necessary in circumstances where quick, accurate changes of focus are important (e.g. bird watching, in a wood).
A focusing lever is not common, but is used, for example, on the Opticron Trailfinder. This mechanism provides very quick focusing while retaining compactness, but can be stiff and overly sensitive to use, and again, ideally needs two hands.
Minox and some others use a slider button, rather than a lever, on low magnification, ultra-compact designs. This slider button is pushed side to side, which is also fast, but sensitive.
Toggle focus is very rarely used (e.g. Carson Bandit 8×25). It provides a one-handed focus mechanism in a relatively large toggle, making it quick and easy to operate "in the field" with gloves, but can be over-sensitive and difficult to fine tune.
The knurled ring around the objective lens appears to be a unique feature of the Minox 8×25 Macroscope and claims to provide quick focusing.
Some low-budget entry-level monoculars from China claim "dual focusing", which means focusing by means of twisting either the main body of the monocular and/or the smaller ring near the eyepiece (referred to as the dioptre adjustment on binoculars). Why dual focusing is felt necessary on a monocular is questionable, but could be for marketing reasons; there is no real technical benefit to such a system, which is never found on the top-quality monoculars from manufacturers like Opticron, Leica, and Zeiss.
As with binoculars, zoom magnification is sometimes available, but is virtually unknown in the best quality units (both binoculars and monoculars) as the optical quality and field of view are seriously compromised. Although zoom systems are widely and successfully used on cameras for observation optics, zoom systems with any credibility are reserved for top quality spotting scopes and come with a very high price tag. Zoom monoculars are available from some "budget" manufacturers, which sound impressive on paper, but often have extreme and unrealistic magnification ranges, as well as an extremely narrow field of view.
Interpreting product specifications.
As mentioned previously, product specifications can sometimes be misleading, confusing or incorrect values stated. Such inaccuracies are more commonly found on budget items but have also sometimes been seen from some brand leaders. For those not experienced in interpreting such specifications, it is always wise to try out the item before buying wherever possible. Some of the descriptors