When you pick up binoculars, you will notice that there are always numbers on binoculars. Do you know what the numbers mean, or do you wonder what they are important for and what do the numbers on binoculars mean?

**The first number** in **10**x42 is the **magnification, **also known as the Power.

**The second number** in 10x**42** is the **diameter of the front** lens given in millimeters.

## What Do The Numbers On Binoculars Mean

The two most prominent numbers on binoculars that you notice first are something like 10×42. These two numbers tell you about the magnification and the objective lens size diameter in millimeters (mm). The numbers are either printed on the body of the binoculars or on the front of the focus wheel.

## Magnification

We want to explain this in more detail using a 10×42 pair of binoculars as an example.

In binoculars, a 10x magnification simply means that the objects viewed in the field of view appear **10 times larger** (or nearer) as seen with bare eyes.

If an object is actually 1000 yards away, at 10 times magnification it looks like it is only 100 yards away. The higher the magnification, the closer things appear. Makes sense, doesn’t it?

## Lens Size – Aperture

The **Objective Lens** of a pair of binoculars has a certain diameter this is also called the **Aperture**, the **diameter is given in millimeters**, hence the 42.

The aperture of the front lens is also called the entrance pupil.

The diameter of the objective lens affects the characteristics of the binoculars. The most important is the light-gathering power, which determines the performance of the binoculars in low light conditions.

## What else do the Number on Binoculars mean?

There are often more numbers printed on the binoculars, which provide further information about the optical properties. One of the most common specifications besides magnification and lens diameter is the field of view.

## Field Of View – FOV

The horizontal width of the image visible in the binoculars is called the **field of view**. The field of view is given in apparent width of the same, in relation to the true distance, e.g. B. 120 yards per 1000 yards.

The numbers for specifying the field of view usually look like this:

**Field 7.1°** or **340/1000yd** or **340@1000yd** or **113 @ 1000m**.

All of these numbers are different ways of expressing how wide the field of view (FOV) is.

### Magnification affects the Field of View

A **lower magnification** results in a much **wider field** of view, which is better for scanning large areas.

With** higher magnification**, the field of view becomes **less wide**, which is better for viewing objects in detail.

## Twilight Factor – The Larger the Objective Lens the better

The objective lens acts as a **collecting lens**, i.e. it collects incident light and focuses it in an intermediate image. The eyepiece enlarges the intermediate image and enables the observer to look at it.

It is obvious that the larger the objective lens, the more light can be collected by the binoculars. Of course, this also means that if more light can be collected, the observed image will also appear brighter.

A large lens diameter means high light-gathering power. Just think of the large telescopes that astronomers use to look into space.

The **light-gathering power** of binoculars is expressed in the **Twilight Factor**, a value derived from the ratio of magnification to objective lens diameter.

**It is the square root of the product of magnification and lens diameter. TF=√M D _{o}**

In our example from above with the 10×42 binoculars, this calculates

- TF=√10 x 42 = √420
- TF = 20.49

In our example of 10 x 42 binoculars, the twilight factor is 20.5

The figure for binoculars is usually between 5 and 25. The higher the number, the brighter the projected images appear in poor lighting conditions.

But it is not only the twilight performance that is key to the usability in low light. In modern binoculars, the highly complex lens coatings increase the light transmission and improve low light performance.

## Exit Pupil

The Exit pupil depends on Magnification and Lens Size. It is the diameter of the projected image that appears in front of the eyepiece. If the observer wants to view the entire projected image, then it is necessary that the observer’s pupil is the correct distance away from the eyepiece ( eye relief – see below).

The Exit pupil can be calculated:

**Exit pupil (EP) = Diameter of Objective lens / Magnification of instrument**

EP = D_{Objective }/ M

With the 10×42 binoculars example as earlier, this calculates:

- EP = 42 mm/10
- EP = 4.2 mm

The exit pupil of our sample 10×42 binoculars is 20.5 millimeters.

The diameter of the exit pupil in binoculars is usually between 2 to 8 mm depending on the parameters of the instrument. The larger the exit pupil, the brighter the image appears. The exit pupil is an important factor for the twilight performance of binoculars.

## Eye relief

Another important number for binoculars is the eye relief of the exit lens. This is particularly important for comfortable viewing

The eye relief is between 10mm and 18mm on most handheld binoculars and means how far your eyes should ideally be from the eyepiece in order to be able to enjoy the full field of view of the binoculars.

The value is especially important for people who have to use their glasses when looking through binoculars, they should make sure that it is least 15 mm or more.

## Close focus

There is another number on the list of binocular specifications, the “close focus”. This number indicates the minimum viewing distance, i.e. how close an object can be to the binoculars in order to still be able to focus on the object.

## What Do The Numbers On Binoculars Mean

Whether for general use, bird or nature observation or hunting, each area of application requires special properties and characteristics of binoculars.

That’s why binoculars come in various sizes with different specifications and different numbers on them. These numbers are important to know which binoculars are best for what type of use.

The two most important numbers are the magnification and the objective lens diameter these say a lot about the properties and types of use of the binoculars.