What to Consider When Buying Binoculars

Did this happen to you? You were thinking of buying some binoculars and, when it came down to it, you didn’t know which ones to take. You’ve been swamped with so many models and features that you haven’t been able to figure out what to base it on in order to know which one best suits your needs. Basically, you have no idea how to choose binoculars, but don’t worry because there’s a solution: we’ve brought you an elaborate shopping guide with all the tips you need to know to make the right choice. Ready?


Binoculars: what are they and what types are there?

The binoculars, also known as a binocular, are an optical instrument made up of two identical tubes inside which are housed a series of lenses, elements and prisms that magnify the image of people, places or objects in the distance.


By looking through them with both eyes open (unlike a monocular or telescope, where you close one eye while looking through the other), they are more comfortable to use and it is easier to see the distance between distant objects and to follow them as they move. This is because the binoculars create the effect of stereoscopy, i.e. the image we see creates the illusion of depth and the scene seems much more realistic and immersive.


Elements of a pair of binoculars

In order to understand how binoculars work, we must first know their internal structure. Binoculars are composed of the following elements:


  • a pair of eyepieces, through which we look
  • a pair of objective lenses, front and closest to the subject under observation
  • a prism, placed between both pairs of lenses


The lenses are the ones that direct the light and focus the magnified image, and the prism is the one that corrects the image so that we receive it correctly, since it arrives upside down.


Other important elements of the binoculars are the focusing wheel, to focus the image, and the diopter correction ring to have a different diopter in one of the eyepieces. Some also include a universal thread so that they can be attached to a tripod.


Types of binoculars

If you’ve noticed, not all binoculars have the same design. While some look more streamlined, others are larger and feel heavier. This difference in appearance and size is due to the type of prism each one uses.


Depending on which prism system the binoculars have, we can distinguish between two types: rooftop and reefer.


As you might have guessed, choosing one prism system or another is going to be one of the first decisions we’ll have to make when it comes to choosing binoculars. As we have already mentioned, each system not only influences the appearance and weight of the binoculars, but also their optical performance.


Porro prisms

The porro prisms, named after Italian engineer Ignazio Porro, are arranged in a non-linear fashion, giving the binoculars that characteristic look with the eyepieces not aligned to the objectives. They are heavier and bulkier, but also more economical and produce sharper, more contrasting images.


Roof Prisms

Unlike joint prisms, roof prisms have the prism aligned with the lenses, hence their appearance is smaller and more compact as the elements are arranged in two parallel straight tubes. They are lighter and more manageable.


How to choose binoculars: things to consider

There is a great variety of models and prices on the market, but deciding on one binocular or another will depend above all on its characteristics and the use we are going to give it.


Materials of the elements

For good image quality, you should always choose binoculars that have mineral (glass) prisms, never organic because they are made of poor quality plastic.


There are two types of mineral prisms: BK-7 (borosilicate) and BAK-4 (barium). BK-7 prisms are cheaper but less efficient at transmitting light. BAK-4 prisms are of higher quality, provide sharper and more contrasted images and are brighter due to their higher refractive index.


Mineral prisms: BK-7 (borosilicate) and BAK-4 (barium)


Optical treatment

When we talk about treatment, we mean lens and prism coatings with anti-reflective layers to reduce or minimize internal reflections and color aberrations and improve light transmission. Binoculars can have the following treatments:


Coated (partial coating) –  one or more surfaces are treated, but the quality is generally poor.


Fully Coated – All surfaces are treated with an anti-reflective coating. It is 80% light-transmitting, with acceptable results.

Multi-Coated – one or more surfaces have several layers of coating, while the rest have only one layer.


Fully Multi-Coated – All surfaces have multiple layers of treatment. They offer a transmission of 90 to 95%, the images are very bright and contrasted.

It is recommended that the binoculars are multi-coated to ensure good image quality.


Magnification and lens diameter

Have you noticed that binoculars use numbers like 7×40, 8×25 or 10×50? The first number refers to the degree of magnification or magnification of the observed object, so in a 7×40 binocular we will see the image 7 times larger than in reality. Above 10x it is difficult to keep the image stable, so it is recommended that a tripod is used.


How to choose a pair of binoculars: degree of magnification


The second figure refers to the diameter of the target lenses, indicated in millimetres. It influences the size and weight of the binoculars and the amount of light they allow to pass through them, so the larger the lens, the brighter the image and the better the quality.


You can also find zoom binoculars, for example 10-30×60, which offer a variable degree of magnification. The advantage of these binoculars is that you can zoom in or out as you please, but they are larger and heavier. Moreover, it is precisely their zoom mechanism that makes them less reliable.


Output pupil

The exit pupil is the cone of light coming out of the eyepiece. The larger the exit pupil, the brighter the binoculars. This diameter is calculated by dividing the diameter of the lens by the number of magnifications, and tells us how much light is reaching our eyes. For example, an 8×40 binocular has an exit pupil of 5mm.


To see the exit pupil, take the binoculars and move them about 25-30 centimetres away from your face: if you look closely you will see two small circles of light in the centre of the eyepieces (diamonds in the case of BAK-4 prisms). To make the most of the binoculars you should place your pupils “inside” these light cones.


How to choose binoculars: exit pupil

The pupil of a human eye varies from 1.5 mm in bright conditions to about 8 mm in darkness. Under normal conditions it is around 5 mm (with the exception of differences between people), so an exit pupil of this size is ideal.


If the exit pupil diameter of your binoculars is smaller than the pupil of your eye, it will appear as if you are looking through a peephole. If it’s larger, you won’t be able to use all the light captured by the binoculars’ lenses. You should also keep in mind that, as you get older, your eyes tend to dilate less, so you should choose an exit pupil that’s appropriate for your age.


The quality of the lenses and the anti-reflective coatings also play a very important role in the luminosity of the binoculars. Thus, between two binoculars with similar optical characteristics, the brightest one has the largest number of exit pupils; but with the same exit pupil the one with the largest opening or the best quality of its elements would win.


Relative brightness

This is the ability of binoculars to capture and transmit enough light to obtain sharp, contrasting images. The bigger it is, the brighter the images will be. It is calculated by squaring the diameter of the exit pupil, so a 7×50 binoculars would have a relative brightness of 50.4.


This index will also depend on other factors such as the diameter of the objective lenses, the degree of magnification, the quality of the materials used and their treatment or the type of prisms.


Twilight factor

If you plan to use your binoculars at night, in addition to the relative brightness, you should take into account the twilight factor. This refers to the ability of the binoculars to recognize details when observing in low light conditions. It is calculated by taking the square root of the product between the diameter of the lens and the degree of magnification. The higher the square root, the better the view of detail even in poor lighting conditions.


Eye Relief

The eye relief is the optimal distance between the eyepiece and the eye so that the diameter of our pupil equals that of the exit pupil and we can see the entire field of vision. A high relief distance allows us to move the binoculars away from our eyes and still see the entire field of view, so the greater the distance, the more comfortable it is to look through the binoculars.


The binoculars have rubber rings to keep your eyes at the correct distance from the eyepieces, so wearing glasses will increase this distance and the quality of the image observed will be affected. Normally 1 cm is sufficient, but people with glasses will need at least 1.5 cm.


However, many models include a dioptre corrector in one of the eyepieces, so that you can adjust the focus and use the binoculars without glasses. There are also adjustable dark circles under the eyes that can be folded back or screwed on and off, ideal if you share binoculars or have a more severe condition.


Angle of vision and field of view

The terms angle of view and field of view describe the visible area covered by the binoculars at a distance of 1,000 metres. The angle of view is expressed in degrees, while the field of view is expressed in meters.


For example, binoculars with a field of view of 100 m, this means that at a distance of 1,000 m, we will cover an area of 100 m. The higher the number of magnifications, the smaller the field of view.


Minimum focus distance

This is the minimum distance that binoculars can focus on an object. It may seem an odd factor to take into account, since the idea of having binoculars is to see things at a distance, but perhaps also at other times we encounter objects located at a short distance and want to see them in detail.


On the one hand, we are interested in keeping it as low as possible but we also have to take into account that the higher the magnification, the greater the minimum focusing distance. If you are looking for a low minimum focus distance, look at the size of the lens and that the degree of magnification is around 8x.



The vast majority of binoculars use a central focusing system. The focusing wheel is located between the two eyepieces and adjusts them symmetrically. Apart from the central focusing, many models include a diopter correction ring in one of the eyepieces (usually the right one) so that we can adjust the focus according to our needs.


Other binoculars have a separate focusing system, where each eyepiece is focused individually to achieve much more accurate results. Finally, there are also binoculars that have no focusing mechanism because they already come with a default focus from a certain distance.



We’re talking about how the binoculars are built. Your mount will determine its resistance to wear and tear and external agents such as rain or dust. They are generally coated with rubber to protect them from shocks and to make them easier to hold.


Waterproofing and impermeability

To ensure that the binoculars are watertight and prevent the optical elements from fogging at low temperatures, many binoculars are filled with nitrogen or argon. These gases ensure that there is no fogging inside even under extreme conditions.


It is also important to consider whether they are waterproof or water-resistant, as this is not the same thing. Waterproof binoculars are 100% waterproof and can be used in the rain with peace of mind, whereas waterproof binoculars have a much lower tolerance level and can only withstand splashes.


If your binoculars are not waterproof you should be careful with rain and sudden changes in temperature.


Ergonomics and weight

Just as we have said before that we do not all see the same thing, we do not all have the same hands, the same strength, or the same resistance. Although we may not realize it at first, these aspects are also fundamental when choosing binoculars, as what may be comfortable for some may be light for others and heavy and tedious.



Collimation is the alignment of the optical elements of the binoculars with respect to their mechanical axis. Good collimation ensures clear and comfortable vision, prevents eye fatigue and the feeling of a double image.


Final conclusions

The choice of a pair of binoculars is conditioned by many factors that you must evaluate before embarking on a purchase. Although the cost is usually the most important factor, bear in mind that nowadays there are many models with an excellent quality-price ratio. As we have already mentioned, the important thing is that it meets your technical requirements and is perfectly suited to the use you are going to make of it.