A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and sometimes prisms; the use of prisms resulting in a lightweight telescope. Volume and weight are less than half those of binoculars of similar optical properties, making it easy to carry, and also proportionally less expensive. Monoculars produce 2-dimensional images, while binoculars add perception of depth (3 dimensions), assuming one has binocular vision.
Monoculars are ideally suited to those with vision in only one eye, or where compactness and low weight are important (eg hiking). However, monoculars are sometimes preferred by those with normal vision, where using both eyes through binoculars causes difficulties.
A monocular with a straight optical path is relatively long; prisms can be used to fold the optical path to make an instrument which is much shorter (see the article on binoculars for details). Monoculars, sometimes called telescopes when used in this capacity, are used wherever a magnified 2-dimensional image of a distant object is required (Though some may be used to look at objects closer).
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 applications.
When high magnification, a bright image, and good resolution of distant images are required, a relatively large instrument is preferred, often mounted on a tripod. Smaller pocket-sized "pocket scopes" can be used for less stringent applications.
Whereas there is a huge range of binoculars on the market, monoculars are less widely available and with less choice in the top quality bracket, with some traditionally very high quality optical manufacturers not offering monoculars at all. Today, monoculars are most commonly manufactured in Japan, China and Germany. Prices range widely, from the highest specification designs listed at over £300 down to "budget" offerings at under £20. (As at Feb 2016).
As with binoculars and telescopes, monoculars are primarily defined by two parameters: magnification and objective lens diameter, for example, 8x30 where 8 is the magnification and 30 is the objective lens diameter in mm (this is the lens furthest from the eye). An 8x 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 4x magnification to 10x, although specialist units outside these limits are available. Variable magnification or zoom is sometimes provided but has drawbacks as will be outlined further and is not normally found on the top quality monoculars. Objective lens diameter is typically in the range 20mm to 42mm. Care is needed in interpreting some monocular specifications which can apply these figures loosely and inaccurately - eg "39x95", which on a small cheap monocular is more likely to refer to the physical dimensions than the optical parameters.
As with binoculars, possibly the most common and popular magnification for most purposes is 8x. This represents a usable magnification in many circumstances and is reasonably easy to hold steady without a tripod or monopod. For viewing at longer distances, 10x or 12x is preferable if the user is able to hold the monocular steady. However, increasing magnification will compromise the field of view (as will be shown below) and the relative brightness of the object. These and other considerations are major factors influencing the choice of magnification and objective lens diameter.
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 (eg 60-90mm). A telescope will be significantly heavier and more bulky than a monocular and due to the high magnifications, normally need a tripod. Most popular monocular sizes mimic popular binoculars – eg 7x25, 8x20, 8x30, 8x42, 10x42.
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:
- Prism type – porro or roof
- Lens & prism coating
- Exit pupil
Exit pupil is defined as the diameter of the objective lens divided by the magnification and expressed in mm. (eg a 8x40 will give an exit pupil 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, especially important in deteriorating light conditions. 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.
|'||Average eye pupil dia change||'|
- Twighlight factor (related to magnification and objective lens diameter and is a guide to the ability to see detail at low light conditions and does not necessarily indicate brightness)
- Transmittance (the % of light transmitted through the monocular, indicating brightness, and will be over 90% in quality instruments)
- Field of view (important in being able to see a wide panorama and not appearing to be looking down a tunnel).
Field of view (fov) and magnification are related; for a given situation, 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 chart below has been compiled by the author to show the fov/magnification relationship based on best-in-class data, taken both from personal tests and from manufacturers' specifications.
- Water/fog proofing
- General construction - material (metal, plastic), types of body coating
- Armoured body protection (to resist knocks and damage in the field)
- Lens protection/covers (some are integrated, some loose)
- Eye relief (very important for spectacle wearers if the full field of view is to be visible. At least 15mm is desirable - ideally 20mm)
However, two additional aspects, which are particularly relevant in the context of monoculars are:
- Focusing mechanism
- Zoom or variable magnification
Binoculars almost universally use a central wheel focussing system, operating on both sides simultaneously. Monoculars, however, use a number of different focussing systems, all with pros and cons. These include:
- A large knurled focussing ring around the body of the monocular
- A small focussing ring close to the eyepiece
- A small external focussing wheel alongside and above the monocular
- A small focussing lever
- A sliding focus button
- A toggle focus mechanism on top of the monocular
- A large knurled ring surrounding the objective lens
The most common is the focussing ring around the body. This retains the compactness of the unit but requires two hands to operate and does not give particularly fast focussing. In some units, the ring can be stiff to operate. The small ring near the eyepiece also 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 wearing gloves. A focussing wheel tends not to be used on top quality monoculars (with the exception of the Bushnell 10x42HD Legend) but is particularly popular on budget offerings from China. Although it makes the monocular more bulky, it does give very convenient focussing with one hand and is particularly fast and smooth, which is necessary in circumstances where quick, accurate changes of focus are important (eg bird watching in a wood). A focussing lever is not common but is used, for example, on the Opticron Trailfinder. This mechanism provides very quick focussing 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, pushed side to side, which is also fast but rather sensitive. Toggle focus is very rarely used (eg Carson Bandit 8x25 ). 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 rather over-sensitive and difficult to fine tune. The knurled ring around the objective lens appears to be a unique feature of the Minox 8x25 Macroscope and claims to provide quick focussing.
As with binoculars, zoom magnification is 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. 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.
Some examples of current monoculars by specification
|Some examples of current monoculars||'||'||'||'||'||'||'||'||'||'|
|price||eye relief||FoV||FoV||FoV||exit pupil||weight||body||body||Comments|
|Leica Monovid 8x20||A||15.0||110||6.3||50||2.5||112||98||36||Comes with close-up lens|
|Opticron 8x42 DBA||A||21.0||122||7.0||56||5.3||343||143||52||Very long eye relief|
|Opticron 10x42 DBA||A||19.0||105||6.0||60||4.2||349||143||52|
|Zeiss Mono 8x20||B||15.0||110||6.3||50||2.5||67||101||?|
|Bushnell 10x42HD Legend||B||15.2||113||6.5||65||4.2||374||137||?||Quick focus wheel|
|Opticron 10x42 BGA||C||16.0||89||5.1||51||4.2||285||136||43|
|Opticron 8x32 LE||C||16.0||131||7.5||60||4.0||272||139||49|
|Opticron 4x12G||C||14.0||219||12.5||50||3.0||49||58||32||"Gallery scope"|
|Opticron 5x30||C||25.0||122||7.0||35||6.0||252||139||49||Very long eye relief|
|Opticron Trailfinder 8x25||D||14.0||119||6.8||54||3.1||131||100||35||Quick focus lever|
|Asika 8x42||E||17.5||121||6.9||55||5.3||329||135||50||Quick focus wheel|
|FoV = field of view (expressed either as m@1000m or as an angle in degrees)|
|Price bands:||A - £250-£350|
|B - £150-£249|
|C - £75-£149|
|D - £40-£74|
|E - < £40|
|Exit pupil = Objective lens dia in mm divided by magnification|
(Prices are typical selling prices as at Feb 2016)
Some monoculars satisfy specialist requirements and include:
- Built-in compass
- Compact, folding monocular
- Night vision system (requiring a power source and usually having low magnification)
- Range finder/graticule 
- Gallery scope (low magnification, wide field of view for use in museums and galleries)
- Microscope conversion & ultra-close focus 
- Built-in image stabiliser
- History of the telescope
- List of telescope types
- Schmidt-Pechan prism
- Spotting scope
- Technical details about monoculars in Germany
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