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Introduction

Magnification is an optical parameter which enlarges/zooms the viewing image and makes the observed object seem bigger. For example, with magnification factor 10 we see objects 10 times larger, which means if an object is 0.1 m high and 100 m away we see it 1 m large. In other words, it’s the same observing the object that's 100 meters away with a 10x binoculars as watching it with the naked eye 10 meters away. In choosing the right magnification for fixed magnification optical products, practice shows that the most useful magnifications are between 7x and 10x, where average people seem to handle optics without too much hand tremor.

Source: ZEISS

Fixed magnification

Optical products with fixed magnification are designed in a way that they allow only one magnification of a viewing object. Due to a smaller number of lenses used in their construction, they are optically brighter and have lower loss of brightness. The number of lenses contributes to its smaller size and lighter weight in comparison to optics with variable magnification. Most of binoculars tend to have fixed magnification, whereas with riflescopes it’s getting rarer each year. Normally this kind of optical products are easier to use and cheaper. They also offer better optical performance, especially in terms of light transmission rate.

Variable magnification

Variable magnification simply means that the optical product is designed in a way where you can change magnification of a certain area. This consequently changes the viewing angle, where higher magnification means smaller/narrower viewing angle and lower magnification means wider viewing angle. Variable magnification adds to versatility and general usefulness.

Lens diameter

Lens diameter represents the second value in product’s name/designation. For example, 10x42 optics have 42 mm diameter of the lens at the front (those that are closer to the viewing object). It is known that the bigger the lens, the more light goes through and the image we see is brighter. All of this, however, depends on the magnification and quality of a certain optical product. Although the bigger lens diameter in binoculars is better, the size adds up on the weight, making it more heavy and difficult to handle. With riflescopes, bigger size of a lens diameter also means more problems with mounting.

The most common lens diameters are 24 mm, 42 mm, 50 mm and 56 mm.

Source: ZEISS

Exit pupil

Exit pupil is a circle from which the light is being transferred to your eye through the optical product. When you holdthe optics a bit far away from your eyes towards a light, exit pupil can be seen as a bright circle in the center of each eyepiece. The larger the exit pupil the more light can reach the eye and the image appears brighter. This is why exit pupil plays an important part in the optical products in poor light conditions at dawn or dusk. Size of the exit pupil also determines how comfortable viewing through an optical product really is. An important factor is also the size of the eye box, which is a space where the eye still has an entire picture, without any tunnel vision or blurry edges. Bigger eye box means more flexibility of the eye position and therefore more comfortable viewing, because the eye can move in several directions within the eye box and still obtain a full image.

Source: ZEISS

The diameter of the exit pupil is calculated by dividing lens diameter with magnification. E. g. 8x50 binoculars have an exit pupil in the diameter of 6.25 mm.

To ensure a brighter image, the eye pupil in low light conditions has to be at least as big as the exit pupil. This way there’s no loss of light and the image is as bright as possible. However, the maximum diameter of the eye pupil depends on age. At night, children’s eye pupils can widen up to 7 mm, while with aging they decrease to a maximum of 4 mm. So if the viewer’s pupils can only be open up to 4 mm, the 7 mm exit pupil cannot be fully utilized. It may contribute to more comfortable viewing, but not to brighter image.

Source: ZEISS

In daylight, when the eye pupil is open up to 3 mm, all the optics with exit pupil bigger than 3 mm are equally bright. For example, the 8x30 binoculars with 3.75 mm exit pupil are no brighter than 8x56 binoculars with 7 mm exit pupil. Those with 7 mm are however more comfortable to use, since they’re less sensitive to the eye position (they have bigger eye-box).

Eye Relief

The only way of obtaining the whole field of view is to have the right distance between the eye and eyepiece – referred to as the eye relief. The eye relief is not necessarily a decisive factor in choosing the right optical product, however it is a useful information if you wear glasses. The eye relief then should be at least 16 mm for comfortable viewing and getting a complete image. Binoculars with too short eye relief give people with glasses a tunnel vision and only show the middle part of an image. With riflescopes the optimal distance is 90 mm or even more for safety reasons (recoil).

Without glasses the distance when using binoculars should be minimum of 15 mm, although everyone has a preferred placement as long as they’re obtaining complete field of view with no blurry edges.

Most binoculars have movable eyecups, where you can adapt them to fit different face structures (if your eyes are further away from you nose etc.) and therefore maintain a steadier grip. If you wear glasses, the eyecups should be closed down so you can lean them onto your glasses.

Source: ZEISS

Field of view

Field of view is an area you see when looking through the optical product. Although it primarily depends on the build of the eyepiece, it is hugely affected by magnification. If you look through two binoculars of the same model but with different magnification, you can see that the one with lower magnification factor will have a wider field of view. So when comparing binoculars, you must compare the ones with the same magnification. With riflescopes the field of view is being measured at 100 m, while with binoculars, spotting scopes and other optical products it's measured at 1000 m.

With binoculars a field of view with more than 140 m at 1000 m distance is considered a wide angle, while with riflescopes it is with a field of view over 38 m at 100 m. Wide angle is particularly useful in bird-watching.

It is also important to mention that the size and lens diameter of optical products are not indicators of their field of view - bigger housing doesn’t automatically mean wider field of view.

Field of view can be expressed in two values – degrees or meters.

Degrees:

One degree is 17.5 m at 1000 m / 1.75 m at 100 m.

If you divide the field of view given in meters by 17.5 you get the field of view in degrees.

Meters:

If you multiply degrees with 17.5 you get the field of view at 1000m.

Source: Lovec

Field of view

Field of view is an area you see when looking through the optical product. Although it primarily depends on the build of the eyepiece, it is hugely affected by magnification. If you look through two binoculars of the same model but with different magnification, you can see that the one with lower magnification factor will have a wider field of view. So when comparing binoculars, you must compare the ones with the same magnification. With riflescopes the field of view is being measured at 100 m, while with binoculars, spotting scopes and other optical products it's measured at 1000 m.

With binoculars a field of view with more than 140 m at 1000 m distance is considered a wide angle, while with riflescopes it is with a field of view over 38 m at 100 m. Wide angle is particularly useful in bird-watching.

It is also important to mention that the size and lens diameter of optical products are not indicators of their field of view - bigger housing doesn’t automatically mean wider field of view.

Field of view can be expressed in two values – degrees or meters.

Degrees:

One degree is 17.5 m at 1000 m / 1.75 m at 100 m.

If you divide the field of view given in meters by 17.5 you get the field of view in degrees.

Meters:

If you multiply degrees with 17.5 you get the field of view at 1000m.

Source: Lovec

Apparent field of view (deg.)

The apparent field of view is a value in degrees that represents the viewing angle of an image you see through the eyepiece. Two binoculars that share the same magnification, lens diameter and field of view don’t necessarily have the same apparent field of view, because it depends on the structure of the lenses inside an eyepiece. It’s simply a subjective impression of the field of view.

Apparent field of view is also depending on the eye relief distance. Shorter eye relief means wider apparent field of view. But if comparing two different binoculars with the same eye relief, the one with the larger eye lens in the eyepiece will have larger viewer field.

It can be calculated by multiplying the actual field of view with the scope’s magnification. Higher value is better as it makes the image appear wider and bigger.

Source: Nikon

Closest focus distance

Closest focus distance is an important value when watching butterflies, moths or plants at a really close distance. It represents the nearest distance where the viewing object can still be in focus. With binoculars, an excellent viewing distance is from 1.5 m below. If you’re not particularly interested in watching objects at a close range this is an irrelevant factor when choosing a new pair.

Filled with

Optical products are often filled with dry gas to prevent the condensation on the inside of the housing when exposing them to temperature extremes. If there is even a slight sign of air inside, there is a certain % of moisture present. Usually they’re filled with either argon or nitrogen gas, which have the same effect – to prevent the moisture and internal fogging without affecting the optical properties. In addition, these gases also prevent the formation of fungus which would destroy the optics. Internal dewing was the biggest problem in older binoculars when exposed to lower temperatures, because they weren’t watertight and contained air. Newer binoculars are therefore all airtight and filled with dry nitrogen or argon.

Waterproof

Waterproof feature is made to keep the optical products sealed and protected from water or dust. Such products are suitable for marine, hunting, hiking or in extreme humidity. Even if you’re not planning on using them in this kind of situations, it is a good feature to have in case of heavy rain or dust. Waterproof optical products are typically sealed with O-rings.

All optical products that are fogproof are also waterproof, because they have to be properly sealed to keep the dry gas inside. Yet not all waterproof products are fogproof as the air inside the barrels is not necessarily replaced with dry nitrogen or argon.

You should be careful not to confuse waterproof with weather-resistant as they’re designed to protect only against light rain and are not fully sealed.

Slightly better waterproofing of binoculars can also be ensured with an individual eye focusing mechanism, due to less moving parts than with the central focusing system.

Fogproof

Fogging in optical products can occur when you move them from the warm insides of your house to the cold outdoors. To prevent the formation of inside fogging they’re often filled with dry gas – either nitrogen or argon which contain no moisture.

To keep the gas intact on the inside, the optics have to be properly sealed, which is why all fogproof optical products are also waterproof.

It’s important to keep in mind that fogproof means that it’s to prevent fogging on the inside of the optics, not on the outside. If your outside surface of the lenses fogs up due to temperature differences or humidity just allow them to adjust back – do not wipe the condensation off as it can be damaging to the glass surface and its coatings.