What do the numbers refer to on binoculars?

Q. What do the numbers refer to on binoculars?

A. Binocular specifications are given in the form of two numbers, for example 8x22 or 10x42.

The first number relates to the magnification of the binoculars, so in the examples above we are looking at binoculars of 8 times magnification and 10 times magnification. To get an idea of what this means in use, imagine that you are viewing an object 500 metres away. When using 8x binoculars the object will appear to be 62.5 metres away as if viewed with the naked eye (500/8 = 62.5), and with 10x binoculars the object would appear as if 50 metres away (500/10 = 50).

The second number on binocular specifications tells you the size of the objective lenses (the ones at the front) in mm. This figure makes it easy to get an idea of the size of the binoculars, and also how much light they let in. In general, a pair of binoculars with 22mm objective lenses will be small and compact, but won't let in as much light as those with 42mm objectives. Binoculars with objectives much above 42mm start becoming fairly heavy, but let in a lot of light.