Binocular Know How – Binocu-huh?

Bino-How gives first time buyers of binoculars the low down on what to look for when making their purchase.

Binoculars come in a confusing range of different sizes and models, but once you know what the numbers mean, it’s really quite simple. The key to choosing a pair of binoculars comes down to being able to determine what ‘field’ conditions you will most often be using them in and consequently, what type of binoculars will be best suited to your needs. Our guide takes you through each of the variables, explaining the terminology and adding useful illustrations along the way to help you make the right decision. We cover it all, from the history of binoculars to the types of models available on the market today, discussing the features and benefits of each along the way. Bino How will clear up any blurring and get you focused!

Binocular types

There are 2 main types of binocular design; Porro Prism binoculars and Roof Prism binoculars. Their descriptions are derived from the layout of the glass prisms within them.

Porro Prism binoculars are the more traditional looking models, with the lenses at one end closer than at the other. Usually it is the eyepieces that are closer together, with the objective lenses at the light gathering end being further apart. However, in some compact models this layout is reversed.

Roof Prism binoculars are essentially two parallel tubes joined together, with the eyepieces and objective lenses at near equal distance apart. This design changes little, regardless of the model size.

Traditionally, porro prism binoculars offered better quality viewing as the layout of the internal prisms allowed more light through and therefore provided a brighter image. Over the years, advances in lens and prism coatings mean that there is now little discernible difference between porro and roof prisms in terms of the light transmission. This combined with the more compact nature of roof prism binoculars means that it is this design which is favoured by most binocular users and manufacturers.

Porro Prism
Porro Prism
Compact Porro Prism
Compact Porro Prism
Roof Prism
Roof Prism
Compact Roof Prism
Compact Roof Prism
Diagram of Porro and Roof Prism Binoculars

Magnification and size

Binoculars are usually described in numeric terms, e.g. 8x42, 10x25, 7-12x50. Although initially confusing, these numbers and their meanings are actually very straightforward.

10x42 10x25 7-15x35 10-15x50

The first number refers to the strength of magnification and how much closer subjects will appear. For example, 8x means that the subject will be magnified by 8x life size and will appear 8x closer. The higher the number, the higher the magnification and the closer the subject will seem. As the magnification increases, the field of view is reduced and the image produced becomes darker.

Sometimes there is more than one number in this part of the description. For instance, 7-12x denotes that the binoculars offer a range of magnifications and can zoom from 7x to 12x including each magnification in-between. Alternatively, 7+12x is a dual function binocular that can offer either 7x magnification or 12x magnification, but not the variations in-between. In general, the sharpness and brightness of images produced by these types of binocular are inferior to fixed magnification models. The dual magnification models are superior to the zoom models and offer a better compromise between convenience and quality, however this comes at a price.

There are limits as to the maximum magnification that can be comfortably hand-held. The higher the magnification, the harder it is to hold the image steady, as every slight tremble is also magnified. Practically, 10x magnification is the highest that can be comfortable hand-held, possibly 12x if you have a very steady hand.

The second number refers to the size of the light gathering end of the binoculars. Quite simply, it is the diameter in mm of the objective lens. The larger this number, the more light will be transmitted into the prisms and the brighter the image will be. In addition, because this is a physical measurement it also gives an indication as to the overall size of the binoculars. A pair with an objective lens diameter of 50mm will be significantly larger than a pair with 25mm lenses, regardless of the magnification.

Leica Range of Binoculars showing the different sizes of different objective lens diameters

Field of view

The design, size and magnification of binoculars all have an effect on the width of the image that you will see through them. As a general guide, higher magnification binoculars have a narrower field of view, but the layout of the lenses and prisms and the size of the binoculars can also affect this.

The field of view will be given in one of 2 ways, either the angle of view in degrees, or as a width of view at a given distance. This second method can either be metric or imperial; metres at 1000 metres or feet at 1000 yards. This diagram shows the same field of view demonstrated in these different ways.

Field of view expresses as an angle or two times the length of the opposite side of a right angle triangle with the accute side at 1000 yards or meters

The field of view of a binocular is dependent on the optical design and not simply a product of the magnification and objective lens diameter. That said - lower power binoculars often have a wider field of view making it easier to locate objects in the field.

Close focus

As well as being used to view far off animals or objects, many wildlife observers also use binoculars to magnify relatively close insects or flowers. As such, the closest point at which the binoculars can focus is important if they are to be used for such subjects. A straightforward measurement of distance, the close focus is in metres or feet. The binoculars will be able to achieve sharp focus anywhere between the close focus distance and infinity.

Close Focusing

Exit pupils

If you hold binoculars at arms length and look at the centre of the eyepiece lens, you can see a small circle of light. This is called the exit pupil. The size of the exit pupil is stated in millimetres and gives a good indication as to the brightness of the binoculars. The larger the exit pupil, the more light will be transmitted from the binocular and the brighter the image will appear. This difference in brightness is only really evident during the evening, early morning, or when light levels are low. In naturally bright conditions there will be no noticeable difference between a pair of binoculars with large exit pupils and a pair with smaller exit pupils.

Exit Pupil example 1Exit Pupil example 2Exit Pupil example 3

In addition to being a physical measurement, the exit pupil can be calculated by dividing the size of the objective lens by the magnification. For example, with a pair of 8x42 binoculars (8x magnification, 42mm objective lens), the exit pupil will be 5.3mm (42 divided by 8).

Using this method makes it simple to compare the brightness of binoculars. A pair of 7x50 binoculars will have an exit pupil of 7.1mm (50 divided by 7) and will therefore transmit more light than an pair of 10x32 binoculars with an exit pupil of 3.2mm (32 divided by 10). Contrastingly, a pair of 10x25 compact binoculars will be of equal brightness to a pair of 8x20 compact binoculars, as both have an exit pupil size of 2.5mm (25 divided by 10, or 20 divided by 8).

The advantage of a larger exit pupil is only really apparent under dim lighting conditions when the user's eyes are naturally dilated. As the diagram below shows, under bright lighting conditions the iris of the human eye contracts and the extra light transmitted through the larger exit pupil is wasted. However, when the iris dilates under dull lighting, the extra light projected from the brighter binoculars is received by the eye and the image appears brighter.

7.1mm Exit Pupil (e.g. 7x50 binoculars)
7.1mm Exit Pupil
Some of the light is wasted under bright conditions.
Most or all of the light is used under low-light conditions.
4.0mm Exit Pupil (e.g. 8x32 binoculars)
4mm exit Pupil
The same amount of light is received by the contracted iris under bright conditions and the dilated iris under low-light conditions.

Although a good indication as to the brightness of a pair of binoculars, the exit pupil size is only one part of the equation. The quality of the glass in the internal optics and the chemical coatings on the lenses can also dramatically affect the brightness of the image transmitted. Compare a high quality pair of 8x32 binoculars with a cheap pair of 8x42 binoculars and the former are likely to give a far better image, despite having a smaller exit pupil. However, the exit pupil size can be a useful guide when comparing different binocular sizes and magnifications within the same model range.

Twilight factor

Some binoculars also give an indication of low-light image detail by quoting the twilight factor. Like the exit pupil size, this figure is reached using the magnification and objective lens size of the binoculars. Specifically, the twilight factor is found by multiplying the objective lens size in millimetres by the magnification and finding the square root of the result. The higher the twilight factor, the more detail will be seen under low light conditions.

The table below shows the twilight factor for different objective lens and magnification combinations.

Objective lens size 25mm 32mm 42mm 50mm
Magnification 7x 8x 10x 12x 7x 8x 10x 12x 7x 8x 10x 12x 7x 8x 10x 12x
Twilight Factor 13.2 14.1 15.8 17.3 15 16 17.9 19.6 17.1 18.3 20.5 22.4 18.7 20 22.4 24.5

If a pair of 8x32 binoculars is used as an example, the twilight factor is 16 (8 multiplied by 32 is 256; the square root of 256 is 16). For a pair of 10x42 binoculars the twilight factor is 20.5 (10 multiplied by 42 is 420; the square root of 420 is 20.5).

Whilst it might be assumed that you will automatically see more detail with a higher magnification, this table clearly demonstrates that the size of the objective lens is of equal importance. More detail will be seen in dim light at 7x magnification with a 50mm lens than at 10x magnification with a 32mm lens.

As with the exit pupil size, the twilight factor is an indication of image detail, but again, the quality of the lenses and prisms is equally, if not more important. As such, many manufacturers do not even quote the twilight factor in their binoculars' specification.


Very few people use their binoculars underwater, so why is waterproofing so important? Even if it's unlikely that they will ever be used in the rain, binoculars with waterproofing have a distinct advantage.

Using your binoculars underwater

When warm or moist air contacts a cool surface, condensation forms. Breathe on the lens of a pair of glasses or turn the fan off in a car on a cold day and you can see this happening. This effect can also cause condensation to form on the inside of a pair of binoculars when they are moved from a cold environment such an air conditioned car or a bird hide on a winter's day, into warm, moist conditions, such as a centrally heated home or outside on a hot sunny day. If the binoculars are being used anywhere particularly humid, such as by the sea or abroad in the tropics, the risk is increased further. At clear up. However worst, mould and fungus can form inside the lens, permanently ruining the binoculars.

Waterproof binoculars are sealed and filled with a dry inert gas which contains no water vapour and can therefore not condense inside. These models are usually listed as "waterproof" or "fog proof", but the terms "nitrogen filled", or "nitrogen purged" also indicate that a pair of binoculars have this quality.

Some manufacturers go so far as to quote a depth underwater to which the binoculars can be submerged without damage. Although not designed to be used underwater, this information is useful if the binoculars are likely to be used on or close to water. Floating straps can be purchased which will hold the binoculars on the surface of the water long enough for them to be retrieved should they be dropped overboard or from a river bank. These straps are usually brightly coloured to aid recovery.

Leica Floating StrapSwarovski Floating strap

Eye relief

This is a measurement in millimetres of how far away the eye can be from the rearmost lens and still receive the whole field of view. Any further than this and the image will start to vignette, meaning the outer edges will not be visible.

The shorter the eye relief, the closer the eye will need to be to the binoculars in order to see the whole image. This can pose a problem for users who wear glasses, as glasses prevent the binoculars from being brought right up to the eye. As the following diagram shows, this can reduce the visible image.

View through the eyepiece With the eyecup extended, the long eye relief makes viewing very comfortable.
View through the eyepiece The long eye relief means that wearing glasses does not reduce the viewed image
View through the eyepiece Shorter eye relief makes viewing less comfortable over longer periods, but the whole image is still visible
View through the eyepiece Due to the short eye relief, when wearing glasses it is not possible to get the eye close enough to the eyepiece to view the entire image.

Image stabilising

As previously stated, the higher the magnification, the harder it is to hold the image still. To counter this, some manufacturers such as Canon, Nikon and Fuji have developed image stabilising binoculars to counter this effect.

Canon IS BinosNikon IS BinosFuji IS Binos

These work in similar way to image stabilised camera lenses and require a battery to power the stabilising motors. As a result these models tend to be much heavier, bulkier and more fragile than non-stabilised equivalents, but much easier to use with higher magnifications. These are often the model of choice for sports viewers who may want the higher magnification but do not require portability.

Without ISWith IS

Binocular accessories

Selection of Rain GuardsRain guards: Although varying in design, rain guards all do the same job. They sit over the eyepieces of the binoculars to protect them from precipitation when being worn round the neck. Rainguards are usually a single item designed to cover both eyepieces simultaneously and will often be rather loose fitting so that as soon as the binoculars are raised for use, they drop away from the eyepieces. Rainguards will usually feature a means of attaching them to the neck strap of the binoculars, either on one or both sides.

Example ClampExample ClampBinocular clamps When it is necessary to use binoculars with a tripod or hide clamp, a device such as a binocular clamp or L-bracket is required to make the connection. A clamp does just that; it clamps to the binocular tube and will have a threaded bush on the underside to attach onto a tripod. An L-bracket also has a threaded bush on the underside to screw on to a tripod, but also has a thread on the uppermost part at 90°. This thread screws into the front of the binoculars' central pivot and is often covered by a blanking plate which needs to be unscrewed first. This facility is not present on all binoculars and is most common on porro prism models, but even then the ability to fix to an L-bracket needs to be confirmed. Some manufactures also make their adapters for attaching their binoculars to tripods. Whilst these are relatively few in number, it is worth being aware of their existence.

After market CaseCases and straps: Although nearly all binoculars will be supplied with cases and straps, there are a wealth of aftermarket replacements that offer increased performance, style or comfort, both from the binocular manufacturers themselves and independent brands.
Leica CaseReplacement cases tend to be more robust than the supplied versions, with hard leather or padded Cordura being the norm. In addition, ever-ready or field holster cases are also available, which either allow the binoculars to be used without entirely removing the case, or allow the binoculars to be rapidly removed with minimum fuss.

StrapThe straps supplied as standard with most binoculars are either very thin, very coarse, or both. As such, replacement straps tend to offer wider, less abrasive support. Neoprene versions also absorb some of the apparent weight of the binoculars making them far more comfortable to carry. As previously mentioned, floating straps are available which will keep the binoculars floating on the surface for a short time to allow their retrieval.

Binocular uses

Usage Recommended Additional features
General Purpose: 8x32, 10x42 waterproofed
Wildlife Watching: 8x42, 10x50 waterproofed
Sports: 10x42, 12x50 Image Stabilised
Low light: 7x50 Image Stabilised
Astronomy: 15x50, 20x80 Image Stabilised
Walking: 8x20, 10x25 waterproofed

Other Binocular Types

Night VisionNight-Vision Very specialised and often expensive, night vision binoculars are relatively low magnification models that will either have a light intensifying function to boost ambient light levels or an in-built infra-red lamp to artificially illuminate a scene. This infra-red illumination is invisible to the naked eye, but can be seen via the battery powered sensor in the night-vision binoculars.

RangefinderRangefinders. Primarily used in various sporting pursuits such as golf or shooting, rangefinders are battery powered and use a light pulse to measure the distance between the viewer and subject. Most can provide the measurement in either metric or imperial, but some brands supply different models depending whether the measurements are required in metres or yards.

MonocularMonoculars. Like a very small scope, monoculars are essentially half a binocular, using just one tube. Because of this they are extremely compact, but only offer 2D viewing.



B or Z
When used on a porro prism binocular, these letters denote the body style (Bausch & Lomb, Zeiss)
When used on a roof prism binocular, B denotes that a full field of view is obtainable for spectacle wearers
Roof prism (Dach)
GA or RA
Rubber armouring
W or WA
Wide angle vision
Centre focusing
Individual eyepiece focusing. Most ‘IF’ binoculars are 7x magnification and have a large depth of focus which tends to eliminate the need for focusing adjustment at distances over 18m
Phase corrected prisms (roof prism only)