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How to Choose Your Binoculars?

Several types of binoculars are available on the market :
  • Pocket Binoculars
  • Stabilised Binoculars
  • Opera Glasses
  • Binoculars for astronomy or navigation
  • Compact Binoculars
  • Night Vision Binoculars
  • Bright Binoculars
  • and more...

What those numbers on binoculars mean?

Let's take the example of "8 x 30" binoculars. 8 is the close-up or magnification factor and 30 is the diameter of the front lens in milimeters.

Before buying any binoculars, it is imperative to ask yourself two questions :

  1. For which use?
  2. How may times a year?

Depending on the answers to these questions, you will be able to move towards this or that type of binoculars:



For which use?

  • Navigation: magnification from 7 to 8 times maximum.
  • Travel: pocket binoculars
  • Wildlife: bright binoculars


How many times à year?

  • 1to 5 times: low-end binoculars
  • 6 to 12 times: mid-range binoculars
  • more than 12 times: high-end binoculars

 

The magnification

The magnification corresponds to the close-up factor of the binoculars. In concrete terms, the magnification is expressed as a factor (3 x, 4 x, 7 x, 8 x, 10 x, ...). Practically, if you look at an object at 100m with 10 x binoculars, it is as if it were seen at 10m.

 

The standard magnifications (6 x, 7 x, 8 x) offer a very large field of view, very good brightness, good image stability and ideal comfort.

High magnifications (10 x, 12 x) offer excellent close-up, however, image stability, field of view and possibly brightness are lost.

The diameter of the lens

The aperture or width of the lens will determine the size of the binoculars as well as its brightness.

The diameter of the exit pupil

This is the round light point that you see when you hold the binoculars at arm's length. Technically, it is the diameter in millimetres of the objective image given by the eyepiece.

The larger the exit pupil, the greater the brightness. But be careful: a large exit pupil is limited by the pupil of your eye. A child will have a 8 mm pupil, a young adult a 6 mm pupil and a 50-year-old adult a 4 mm pupil.

The exit pupil diameter is calculated by dividing the lens diameter by the magnification. For example:, binoculars 10 x 50, the exit pupil will be egal to 50 / 10 = 5 mm.

 

The field of view

This is the width of the landscape observed through the binoculars, measured at 1000 meters from the observer. The bigger it is, the more comfortable the observation. It is expressed in meters (m) or degrees (°). e.g. 114 m, 6.5°

The Optical Treatment

Almost all binoculars optics are treated with an anti-reflective coating that improves their transparency. But there are different qualities of anti-reflective coatings.

The weight

This is an important element to consider for hiking and long observations.

The Brightness

The brightness is dependent on the binoculars lens diameter and magnification. The larger the lens diameter, the brighter the binoculars are at the same magnification. The best way to determine the brightness of an instrument for daytime use is to calculate the exit pupil diameter (dividing the lens diameter by the magnification).

Warning: the twilight index does not mean anything at all, because it is a theoretical calculation that varies according to the ambient light and the age of the observer. The relative capacity of light transmission in binoculars (twilight index) is calculated by taking the square root of the instrument's magnification multiplied by the diameter of the lens. Using this calculation to prove the brightness of an instrument can be misleading because from this mathematical formula one would conclude that binoculars of 20 x 70 are less bright than binoculars of 30 x 70, which is false.

The Eye Clearance

The Eye Clearance is the distance between your eyes and the first lens of the eyepiece. A long eyepiece clearance allows for more comfortable observation, especially when wearing distance glasses.

 

The Optical Quality

To determine the optical quality of binoculars, the distortion of the image, the sharpness at the edges of the field, the colour rendition and the sharpness of the image must be assessed.

 

The Construction

There are two types of binoculars construction. In binoculars there are three optical components, two eyepieces, two image rectifying prisms, two lenses.

 

One of the constructions uses Porro prisms; it differs by the fact that the lenses are not in the axis of the eyepieces, but offset.

The other solution uses roof prisms, which allows the lenses and eyepieces to be on the same axis. This solution makes it possible to make compact and bright binoculars, which is very important for hiking or long observations.