What Do The Two Numbers On Binoculars Mean

There are several numbers on binoculars. The most prominent two numbers on binoculars provide information about the magnification and the lens diameter. From this, and a couple of other specs, you can determine the optical performance and know what the binoculars are best for.

What Does 10×42 In Binoculars Mean

what do the numbers on binoculars meanThe numbers that are usually most prominent on a pair of binoculars provide information about the magnification and the diameter of the objective lens.


The first number in 10x42 is the magnification, also known as the Power.  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 second number in 10x42 indicates an objective lens diameter of 42 millimeters. This parameter is also called the entrance pupil. The aperture of the objective is crucial for a number of factors. The most important point is the light-gathering power, which determines the performance of the binoculars in low light conditions.

What else do the Number on Binoculars mean?

lower magnification vs larger magnification

Field Of View – FOV

field of viewThese numbers are also frequently found on binoculars: 340/1000yd or 340@1000yd or 113 @ 1000m.

This number refers to the Field of View FOV.

Magnification affects the Field of View

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.

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.

The larger the diameter of the lens, the more light can be collected, thus the brighter the image will appear.

The objective thus affects the light intensity of the instrument, with binoculars one speaks of Twillgt Factor.

The twilight factor expresses the performance of the binoculars in low light conditions.

It is the square root of the product of magnification and lens diameter. TF=√M Do

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.

The twilight factor alone is not enough to assess the twilight performance of a pair of binoculars; the transmission of the optical glass and its coatings matter, as well as the exit pupil.

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 = DObjective / 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

The eye relief for binoculars is also given as a number listed in the specifications. It is usually a value between 10mm to 18mm and describes 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 particularly important if you wear glasses, then it should be 15mm or more.

Close focus

There is another number, the “close focus” on the list of the binoculars specification. This number is usually a value from 10 feet to about 20 feet depending on the type of binoculars. It indicates the minimum distance how close an object may be to the binoculars in order to still be able to focus on an object.

What Do The Numbers On Binoculars Mean

Binoculars are available in a wide variety of designs and specifications. Be it for general use, bird or nature watching, or hunting, each area of application requires special features and ratings from binoculars. The two main parameters of binoculars are the magnification and the lens diameter. These two values can reveal a lot about binoculars.