Binocular designations

To be honest, I’ve been putting this one off for a while because the mathematics turns my brain into a Möbius strip. I’ll try to keep it understandable!

One of the common questions that people buying binoculars ask is “what do these numbers mean?”

The important ones are the “8x42”, “20x100”, or others in the photos. These tell you the magnification and aperture of the binoculars.

Magnification and aperture

Magnification, the first number, is pretty simple. If you’re looking at something through a pair of 10x binoculars, whatever you’re looking at will appear one tenth of the distance away.

The aperture is the diameter of the big lens at the front of the binocular, measured in millimetres. It relates to the light-gathering ability of the binoculars.

On a bright day, this may not be important. But if you’re hunting owls or star gazing, it's critical. The more light you get in, the more you can see.

Magnification and aperture interact as well. The more magnification the binoculars squeeze out of the light coming in, the dimmer the image becomes. So at night, an 8x binocular will be brighter than a 10x binocular.

Field of view

The Celestron Upclose 20x50 has three numbers: 3.2°, 168ft and 56m. These are three ways of saying the same thing.

Imagine you’re standing near a railway line – how long is a train that just fills your field of view?

Let's stand 1000 metres away from the track and measure the train in metres. For the Upclose 20x50, a train 56 metres long will just fill the binocular’s field. Obviously, if you use a lower magnification binocular (such as the Noctivid 8x42) you’ll be able to get a much longer train into the field.

 

You can safely ignore the 168 feet measurement, that’s for people who use the imperial system.

But the angle in the diagram? The length of the train divided by the distance to the railway is equal to the tan of the angle.

That’s the trigonometry. We're back to the Möbius strip!