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1 October 2020

Binocular basics

Binoculars are often described in baffling jargon, making it hard to know which ones to buy. We take you through the basics to help you decide.
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WhichEditorial team

Choosing binoculars is easier once you understand the main features and how they impact on performance.

Binocular types

There are two main types of binocular design: roof prism and porro prism. Each type differs in the way the prisms channel light through the binoculars to your eyes.

  • Roof prism binoculars have an ‘H’ shaped design, where the eyepiece and the binocular tubes are in a single line. Roof prism binoculars tend to be more compact.
  • Porro prism binoculars have a traditional ‘M’ shape design, where the eyepiece and the lens are not in line. 

Specifications of binoculars

Nearly all binoculars will have two numbers written on their casing, like this: ‘8x25’ or ‘10x42’.

1. Binocular magnification

The first number is the binoculars’ magnification. This tells you how many times larger an image will appear compared with that seen by the naked eye. A magnification of ‘8x’ means that the object you observe with binoculars will appear eight times closer than it does in the real world.

How much magnification you need depends on what you're trying to do with the binoculars. If you want to watch sport, for example, then a wider 'field of view' is more useful than strong magnification. For horse racing, for instance, a wide view of the action is a lot more useful than a close-up of a horse’s head.

If you aim to use the binoculars without a tripod, then models with an 8 x magnification are generally easier to hold steady than those with a larger magnification.

2. Objective lens diameter

The second number refers to the objective lens diameter. This is the lens through which light enters the binoculars. The larger this number, the brighter the image in the binoculars will appear (all other factors being equal). The larger the objective lens diameter, the larger and heavier the binoculars are likely to be.

Generally, binoculars with an objective lens diameter greater than 30mm are classed as standard-sized binoculars. Those with an objective lens diameter of less than 30mm are classified as compact binoculars.