What Do The Numbers On Binocular Mean?

What Do The Numbers On Binocular Mean?

Monoculars and binoculars are designed to observe from a distance and, like any instrument, have their own units of measurement and technical terms that may seem difficult at first.

Whether it is to cultivate a new passion, take away a curiosity or get more information in anticipation of a purchase, I am sure that at least once you have thought “What do the numbers that appear on the binoculars mean?”

In this article you will learn what the numbers that appear on the monocle or binoculars mean, how to choose the right model for you, what are the characteristics to look for, the main technical terms used by manufacturers and you will discover useful tips for professional observations.

What Do The Numbers That Appear On The Binocular Mean?

To refer to monoculars and binoculars and distinguish one model from another, two digits are used: for example 12 × 50 mm or 10 × 42 mm.

The first number, usually called magnification, indicates the power, or how many times the observed images appear magnified. A 12 × 50 mm binocular will show us the objects (or subjects) 12 times closer, while a 10 × 42 mm will have a power of 10 times.

The second number after the x is expressed in millimeters and indicates the diameter of the objective lens. A 12x50mm binocular will have a 50mm diameter objective lens, while a 10x42mm will have a 42mm lens.

1. Magnification

The magnification is the first number of the binoculars and indicates how many times the images are close together.

Greater magnification of the image = Lower angle of view

Binocular magnification

What does it mean?

If the image appears larger to our eyes, the angle of view (i.e. the space to be framed) is narrower. When observing with the binoculars, the vibrations of the hand can make the framed image unclear. With magnifications greater than 10x, a tripod is generally used to have a stable monocle and clear and detailed observation.

If, on the other hand, you are looking for a more panoramic observation and fewer details to frame, choose a lower magnification.

2. Lens Diameter

The second number after the x indicates the diameter of the objective lens expressed in millimeters which determines the amount of light collected and the size of the field of view. This feature is important in low light conditions such as during sunset or twilight.

Binocular lens diameter

Larger lens diameter = Greater brightness and visible details = Greater weight of the binoculars

The larger the lenses, the heavier the binoculars (or monocle) will be. Usually for travel, birding and outdoor activities, binoculars with diameters not exceeding 50 mm are indicated as they are light and practical to carry. For trekking and day observations, a diameter of 20 mm is sufficient while for night observations, a diameter of 50 mm guarantees greater visibility.

3. Let’s Make An Example

What is the difference between a 10x42mm binoculars and a 12x50mm binoculars?

In the first case we have a magnification power of up to 10 times and with an objective lens diameter of 42 mm. In the second case we have a binoculars that allows us to see 12 times closer and with a 50 mm objective lens.

Binocular on a tripod

By comparing the diameter of the objective lens, the second will collect more light and the size of the field of view will be wider than the first. To maintain a more stable and clearer observation, in the second case we will need a tripod. Knowing what the numbers that appear on the binoculars mean are not enough. Keep reading to discover other important features to consider.

4. Field Of View

The field of view represents the area that you can observe with a binoculars at a distance of 1000 meters and can be expressed in both meters and degrees.

For example:

In the technical data sheet of a binoculars you can read Field of view 122M / 1000 M, it means that at a distance of 1 km, you will be able to observe a field of 120 meters.

You may happen to read “Visual angle”, also in this case we refer to the area that we can observe with the binoculars but this time expressed in degrees. We can calculate it with a mathematical formula where the factor 17.5 represents the visual field in meters corresponding to a visual field of 1 °.

Binocular field of view

Field of view at 1000 m = Angle of view in degrees x 17.5

Viewing angle in degrees = Field of view at 1000 m / 17.5

Let’s take our 12 × 50 mm binoculars with 120 M / 1000 M field of view and apply both formulas to calculate the field of view and the angle of view.

Field of view at 1000 M = 6.85 x 17.5 = 120

Visual angle in degrees = 120 / 17.5 = 6.8 °

In summary, the binoculars provides a field of view of 120M / 1000M and has 6.8 ° of field.

The width of the field of view is inversely proportional to the magnification: a binoculars with 10x will have a greater field of view than a 12x binoculars.

Lower magnification = Greater field of view

5. Output Pupil

Another value to consider when buying a binoculars is the exit pupil expressed in millimeters which indicates the diameter of the light beam that reaches the eye.

Exit pupil = Lens diameter / Magnification

Binocular output pupil

Let’s take our 12 × 50 binoculars, the exit pupil will be 4.5 (50/12). A normal value that guarantees clear daytime images and good images in low light conditions.

The exit pupil is a parameter to be taken into consideration since it should be at least as wide as the pupil of our eye, which varies from 2 mm in bright light to 7 mm in low light conditions.

Greater exit pupil = Greater light transmission = Brighter images

Now everything is clearer, for nocturnal observations, it all depends on the values ​​shown in the data sheet. The next time you read the exit pupil value, you can get an idea if the binoculars is also suitable for twilight or sunset observations.

6. Eye Relief

Eye relief is different from the exit pupil, although apparently the terms may seem synonymous. Eye relief is the distance between the eyes and the lens of the monocle and varies from 5 to 20 millimetres.

It is especially important for eyeglass wearers to understand broadly if that model can fit. When we wear glasses, the eyes are further away from the lens and we risk losing the image. If the distance between the eyes and the lens (ie the eye relief) is greater than 17 mm, it means that even in the case of glasses, the monocle will allow you to see the same field of view.

best birding binoculars

Greater eye relief = Better observation with glasses

The ideal eye relief for those who wear glasses is 20 mm and is also perfect for viewing with the naked eye in order to observe clearly and blink freely.

Our binoculars, for example, has a 20 mm eye relief and through the dioptric adjustment wheel you can adjust the focus according to your myopia.

7. Types Of Lenses

The lenses used for monoculars and binoculars can be made in:

– Plastic. They are appreciated for their impact resistance during extreme activities. The images produced have a lower quality, consequently the price is lower.

– Glass. They are mostly used in professional monoculars and binoculars. They produce quality images and the price is higher. The glass reflects the light that enters from the outside but this phenomenon is minimized through an anti-reflective treatment on the lens.

Binocular types of lenses

Abbreviations are used to distinguish the different types of treatment for better light transmission and ensure clear images:

C (Coated): The outer lenses have been coated with a single layer of antireflection

MC (Multi Coated): The external lenses have been coated with multiple layers of anti-reflective treatment which offers better light transmission;

FC (Fully Coated): Both the prisms inside the binoculars and the external lenses have been coated with a layer of anti-reflective treatment;

MFC (Multi Fully Coated): Both the prisms inside the binoculars and the external lenses have been coated with multiple layers of anti-reflective treatment for better light transmission and greater protection of the lenses from water, humidity, scratches and accidental bumps.

Multi-layer FC and MFC coatings are superior to those with a single layer and also affect the higher cost of monocular and binoculars.


There are no more secrets for you in the technical data sheets of monoculars and binoculars. While not trying it personally, now you know what the numbers that appear on the binoculars mean and you know all the features to take into consideration to buy a model that is right for you.

We hope to have helped you with this guide to understand what is important in binoculars, now it’s your turn to pick the best one to suit your needs!