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Monocular

Optical device

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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 ends and a telescope starts 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.

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