• There are quite a few definitions and values about binoculars. What types are there? What do all these terms mean? In this binoculars tutorial, we try to make it all a bit clearer. Compiled by our binoculars experts Jan and Maarten. They have visited several factories and are well versed in the world of binoculars, spotting scopes and other equipment.

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    However, we also understand that there are many different binoculars and concepts. This is why we are happy to provide you with personal, no-obligation and, above all, free advice. How convenient is that! By sending us a message via the contact form, we can help you make the best choice. We often respond the same working day (provided you send us an e-mail within our opening hours). In order to make the right choice quickly, it is useful to mention in your e-mail what you are going to use the binoculars for.

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Types of binoculars

There are many types of binoculars. Most binoculars are binoculars. These binoculars have two equal optical systems next to each other. They allow you to see with two eyes. You also have monocular binoculars. What types of binoculars are there?

Binoculars consist of a lens and an eyepiece and are equipped with a system of prisms to straighten the image (an astronomical telescope without these prisms gives an upside-down image). Binoculars are therefore also called prism binoculars.


Binoculars can be divided into two types; porro and roof prism binoculars. The difference between the two is the arrangement of the prisms.

Roof prism binoculars

In roof prism binoculars, the roof prisms (also called pentaprisms) are placed one behind the other. The light passes through the binoculars almost in a linear line. This allows this type of binocular to be more compact.

Porro binoculars

With porro binoculars, the image is transported via an N-shaped bend, whereby the prisms and the objective and eyepiece are not directly opposite each other, which is why this type is generally wider than roof prism binoculars. The advantage of this arrangement is that binoculars with a porro prism offer somewhat greater depth perception.

Best choices lists


  • Best choices for monoculars

  • Monoculars are viewers which allow you to see with only one eye. They are chosen when weight and size are important. Very handy to put in your inside pocket, but also for visually impaired people who want to look at the blackboard in the classroom. Monoculars are usually constructed in the same way as roof prism binoculars.

What do all those numbers about binoculars / spotting scopes mean?

Magnification / Objective diameter

There are always two numbers on binoculars. For example, the numbers 8x40 on binoculars means that the binoculars have 8x magnification and a 40 mm objective diameter, this means that the objects you are looking at are brought 8x closer, and the objective lenses (the lenses on the front of the binoculars) are 40 mm in diameter. If the diameter is larger, the binoculars will let in more light, and you will have a brighter image. The term diameter objective lenses is also referred to as effective aperture of the front lens.

Exit pupil

If you hold binoculars up to the light at a distance of about 30 cm from the eye, a circle of light will be visible in the eyepieces (the lenses facing the eye). The size of the exit pupil varies according to the model of binoculars. The size of the light circle can be calculated by dividing the objective diameter by the magnification. So, for 8x42 binoculars, the exit pupil is 42/8 = 5.25 mm.

In young people, the pupil of the human eye can open to about 7mm in the dark, and closes to about 2mm in bright light. As people grow older, the pupil flexibility decreases and it only opens to about 5mm in the dark.

The above shows that there is no added value in buying binoculars with an exit pupil larger than 8mm, as the human eye cannot use the larger spot of light.

The shape of the exit pupil is also an indication of the quality of the binoculars. If the exit pupil is perfectly round, this indicates very good lens quality. If the lenses are of poor quality, the exit pupil is not perfectly round, but angular or misshapen, resulting in reduced light output and poorer image quality.

Twilight value

The twilight number is actually a kind of measure of the performance of the binoculars as a whole. The rule is: the higher the better. The higher the twilight number, the better the performance/details in unfavourable light conditions.

Again, this calculation does not take into account performance-enhancing devices such as coatings and the use of special types of glass. The twilight number is obtained by multiplying the magnification and the objective diameter, and then taking the square root of this product.

So for an 8x42 binocular, 8x42=336. Then we take the square root of 336 = 18. 3 (rounded). When multiplying the twilight number by 10, we get the number of metres at which details can still be discerned in unfavourable conditions. In our example of the 8x42 binoculars, this becomes 183 metres.

Brightness (geometric) / Relative brightness

Luminosity refers to the amount of light transmitted by the binoculars. The luminosity is expressed as a number. The luminous intensity is the square of the exit pupil, in other words, the luminous intensity is obtained by multiplying the exit pupil by itself. For 8x42 binoculars, the exit pupil was 5.25, so the luminosity becomes 5.25x5.25 = 27.6 (rounded off).

In principle, the higher the better, but here too we know that a luminosity of more than 49 (=7x7) no longer has any added value. Binoculars with high brightness (7x50, 8x56, 9x63) are also called night vision binoculars and are mainly intended for hunting.

The above calculation does not take into account any performance-enhancing modifications to the glass. By applying better types of glass and coatings, a better luminosity is obtained. The geometrical brightness does not say anything about the actual brightness of the binoculars. This makes it impossible to compare binoculars on the basis of brightness alone. 

Light transmission (%)

Light transmission is the only value by which one can judge the optical quality of binoculars. Nor is it possible to determine the light transmission with a basic theoretical formula. When measuring the light transmission, one also takes into account the quality of the glass, the coatings and the construction of the binoculars. Measuring the light transmission is done with special equipment. However, this value is not mentioned with every pair of binoculars. High-quality binoculars do mention the type of coatings on the lenses. 


Coatings prevent the reflection and scattering of light. This reduces light loss and improves contrast. Untreated glass can lose up to 5% in light transmission. Binoculars consist of several glass lenses and would therefore lose a lot of light with untreated glass.

A single layer of anti-reflection coating can reduce the loss by about 1.5%. Through multiple layers of different coatings, the light loss can be reduced to as little as 0.2%. 

A coating is applied to the glass by evaporating the coating. This happens in a protected environment so that no dirt can come along with the coating. A damaged coating cannot be repaired.

Eye relief

The maximum distance the eye can be from the eyecup to see the entire visual field. This is important for spectacle wearers, because with spectacles they are somewhat further away from the eyecup. If the eye is further away from the eyecup than the maximum distance, you see less.