• Different types
  • Prism
  • Mystery about the numbers
  • Field of view
  • Coating
  • Glass 

Different types 


Binoculars feature an eyepiece where you can use both eyes ('bi') at the same time. You will always find two numbers; for instance 8x42. These two numbers stand for the magnification and the diameter of the objective lens, respectively. The higher the magnification, the more the binocular zooms in on the object or animal you are looking at. The diameter of the objective lens determines the amount of light entering the viewer and therefore the brightness of the image. The size of the diameter matters especially in when delaling with poor light conditions (e.g. during twilight or a cloudy day). From the magnification and the diameter, the exit pupil, twilight number and light intensity can be derived.

binoculaire verrekijker


Apart from binoculars, there are also monoculars where you use only one eye ("mono"). The big advantage of monoculars is that they are very compact and therefore handy to carry around in your pocket. However, for long-term use we recommend the binoculars, since they are more comfortable to look through. In addition, monoculars do not provide as much depth.

Spotting scope

The spotting scope is like a monocular but with a much higher magnification. They are therefore ideal for distances of 100+ metres. Because of the higher magnification, it is important that the scope is on a tripod, so the image does not move too much. The spotting scopes are quite large and therefore less mobile. If this is a problem, there are also the more compact 'mini' scopes; lightweight and easy to carry in your backpack.


A prism is a transparent optical element, consisting of several polished flat surfaces, which refracts light rays, forming a light spectrum, and projects the image upright. For binoculars, there are two types of prisms; the roof prism and the porro prism


Roof prism binoculars are easily recognisable by their streamlined H-shape, where the axes of the eyepiece and objective lens are in a straight line (one tube). This type of binoculars is therefore much more compact. Together with the internal focus system it is also easier to make the binocular dust- and waterproof. To guarantee high quality, the roof prisms have to be produced very precisely (higher precision) and have to fit well into the coating. This is why the average roof prism binocular is a bit more expensive than the average porro binocular.  


With porro binoculars, the eyepiece and objective lens are offset from each other so that they are not in one straight line. This offset adds depth to the image, but also means that Porro binoculars are always larger and heavier. They also have an external focusing system (displacement of the eyepiece), which makes it more difficult to make the binocular waterproof and makes it more sensitive to dust and knocks. The production of the porro prism is less complicated and therefore this type of binoculars offers a good quality for a lower price. 






Compact and light


Continuously in development

More depth



Generally more expensive 


Heavier and more clumsy

Less sustainable


The mystery behind the numbers [e.g. 8x42]

vergroting verrekijker

For binoculars, monoculars and spotting scopes, we keep seeing two numbers. But what do these numbers tell us about the device?

Simply put, the first number indicates the magnification factor of the binoculars and the second number the diameter of the objective lens (in millimetres). The magnification factor says nothing other than the extent to which your view is enlarged by the type of binoculars. In other words, with a magnification of 8x, an animal at a distance of 800 metres will appear to be standing at a distance of 100 metres from you (distance/magnification factor). Now you may think, the higher the magnification, the better the binoculars. If only it were that easy. There are a number of other aspects that influence the quality of the binoculars, including

  • diameter of the objective lens (read below);
  • coating of lenses and prism;
  • type of glass (e.g. ED, HD, XD).

For the right magnification, you should consider what you will be using the binoculars or spotting scope for and for what kind of distances. For universal use, a magnification of 8x is sufficient; it can be used for both sporting events and birdwatching trips. To get an even better view of an animal, you could consider a 10x magnification. Be aware, however, that this also narrows the field of view and slight hand movements become more visible. For real close-ups, it is better to look at spotting scopes. Due to the high magnification (≥20x) - and the fact that they are less easy to handle - a tripod is a requirement. Spotting scopes are therefore often used to observe the environment from a fixed spot.

The second number represents the diameter of the objective lens (in millimetres). The rule is: the larger the number, the more light enters the scope and therefore the better the image quality during poor light conditions (e.g. dusk, clouds). This also means that the viewer feels a bit heavier. To get a good idea of the total light power of the viewer, you can derive the exit pupil, the twilight number or the light intensity from the diameter of the lens, together with the magnification factor (see below).

Exit pupil

Keep the binoculars about 30 cm away from your eyes and look through the eyepiece. If all goes well you will see a small light circle of a few millimeters in each eyepiece. This is the exit pupil. In humans, the diameter of the eye pupil varies approximately between 2 and 7 mm, depending on the person’s age, health, etc. In good light conditions, the eye pupils are small. With decreased light the pupils will become larger, allowing more light to be absorbed by the eye. This is also the case with binoculars or a spotting scope. A larger exit pupil provides a higher light sensitivity, so more details become visible during twilight or a cloudy day. To ensure a clear image during reduced light conditions, we recommend a minimum exit pupil of about 4 to 5 mm. This value can easily be calculated (see formula). For example: a 8x42 binoculars have an exit pupil of 5.25 mm (=428)


Twilight number

The twilight factor also says something about how well binoculars perform under poor light conditions. For this consider a minimum value of 15, if you want to have a clear image during twilight. This value can easily be calculated using the formula. For example: a 8x42 binoculars has a twilight number of 18.33 (= (8* 42)).


Light intensity

Then there is the light intensity, calculated using the exit pupil. Also this you can easily calculate using a formula (see above). For example: a 8x42 pair of binoculars has a light intensity of 27,56 (=5,252). For this, a minimum value of 16 is desirable. 


The exit pupil, twilight factor and light intensity provide you with a good indication of the light output of the binoculars, monoculars and spotting scopes. However, when comparing it is important to also pay attention to the type of coating and glass. They determine the light transmission of the binoculars, which is very decisive for the quality of the image. Unfortunately there is no simple calculation to measure this. However, we – WMS – pay very close attention to this and all the products in our range are provided with high quality coatings and glass.

Field of View (FOV)

The field of view (FOV) is either expressed in degrees or by the width in meters from a fixed distance (often 1000 meters). The wider the field of view, the easier you get to spot an animal or other object. This can be very useful when observing fast-moving animals, such as birds. A narrower field of view can be very useful to observe an animal or object in more detail.  



Coating| Makes the difference!

When looking through a binocular, only part of the incoming light reaches your eyes (with uncoated glass sometimes up to only 50%), since the glass elements (objective, prism and eyepiece) partially block the light. To reduce the reflection of the light on the glass surface and to increase the light transmission of the lenses, a coating is applied to the lens. This is an extra chemical layer on the lenses that ensure a clear, colorful- and contrasty image. The better the coating, the better the light transmission and thus the better the image quality. Of course, the price range also increases. There are several type of lens coatings, namely:

  • Coated’ (C): single layer on one of the lenses.
  • Fully coated’ (FC): single layer on all the lenses.
  • Multi-coated’ (MC): multiple layers on one or more lenses.
  • Fully multi-coated’ (FMC): multiple layers on all the lenses.

Also the roof prisms are coated with a so-called phase coating; a thin layer of dielectric material. This type of coating slows down the light waves of incoming light and ensures a good connection between the peaks (in 'phase'). With a roof prism, part of the light returns, causing the peaks of the light waves to no longer match each other properly causing the brightness of the image to decrease. A phase coating offers the solution for maintaining a proper brightness, hence image quality.

Cleaning tip!

When cleaning the binoculars, it is important to consider the coating on the outer parts of the lens. You can easily damage the coating with a dirty cloth or your fingers. A special cleaning kit with a cloth and a special lens cleaner is therefore highly recommended!  


In optical instruments, and therefore also binoculars, the quality of glass determines the quality of the images. High-quality, processed glass prevents light dispersion, color fading (spherical aberration) and unwanted reflections. There are many different types of glass to choose from. It is mainly crown glass and flint glass that is used for the production of optical lenses and prisms. The composition and quantity of the ingredients (e.g. barium, calcium, fluoride, potassium and lead) determines the quality and optical properties of the glass, such as light transmission, refraction, reflection and absorption.   

BK7 & BaK4 (prism)

The prisms in the binoculars usually consist of BK7 or BaK4 crown glass. BK7 is an excellent type of glass that is often used for binoculars within the cheaper and average price range. BAK4 is a somewhat more expensive type of glass to produce and is therefore more likely to be found among the high quality binoculars. BAK4 has a higher light transmission which provides more detail and clarity.   

Test it yourself! You can easily find out for yourself whether the binoculars is equipped with BK7 or BaK4 glass. Hold the device one arm length in front of your face and look at the exit pupil. When you see light-dark shades on the side you are dealing with a BK7 type of glass. When it looks tight and round, without light-dark tones on the side, it is a BaK4 type of glass  

ED & HD (lenses)

SD and HD glass is primarily used for the better- and high-quality binoculars. The complex ED glass ('Extra-low-Dispersion') has a very low dispersion with a reduced color shifting, creating an exceptionally detailed, colorful and contrasty image. For this reason, ED glass is also called HD glass ('High Definition'). The more regular HD glass also delivers fantastic quality! Differences will therefore hardly be noticed by the more inexperienced viewer.