Binoculars jargon buster
Binoculars can often be described in baffling language. While you don't need to know everything about their anatomy, an understanding of some of the key terms and features will help you get the best out of your binoculars.
Apparent field of view
A set of binoculars is considered ‘wide angle’ if your apparent field of view is greater than 65 degrees.
Central focusing wheel
The central focusing wheel is used to bring an image into focus for both eyes equally. It should be used in combination with the dioptre adjustment.
This is the shortest distance to achieve an image in focus. The distance is measured in metres.
Collimation refers to how parallel the two binocular tubes are. The more parallel, the less likely it is that you’ll get eye strain or a double image.
The dioptre adjustment compensates for the difference in the strength of your eyes, and helps you to get the clearest images. It should be used in combination with the central focusing wheel. Visit our advice page to make sure you get the best out of your binoculars.
Field of view
This describes how wide an area you can see through the binoculars. Usually measured in degrees, it is sometimes also written as a number. That number will be the width of the scene visible through the binoculars from 1,000 metres away. The greater the field of view, the wider the image you can see. Generally, the higher the magnification, the smaller the field of view. However, specially manufactured ‘wide-angle’ binoculars can give a field of view greater than those with a comparable magnification.
The interpupillary distance on a set of binoculars defines how far apart the left and right barrels should be to fit your eyes. On most binoculars, you can adjust this using a hinge mechanism. As the hinge moves, the two circles you should see in the right and left eyepieces will gradually merge into one. When you can see only one circle, the interpupillary distance is correctly set.
The light that enters your binoculars can be reflected many times through a set of lenses before it reaches your eye. If your lenses are not ‘coated’, each time light is reflected, some light is lost. As well as making the final image darker, the 'lost' light will reflect inside the binoculars, confusing the final image. Good-quality lens coatings will reduce surface reflections and improve light transmission. This will help to produce a brighter image (all other factors being equal). There are four main types of binocular lens coatings, with fully multi-coated lenses being the most effective. From least to most effective, these are the coatings available:
- Coated Some surfaces, but not all, are coated.
- Fully coated All air-to-glass surfaces are coated, but probably not any plastic surfaces.
- Multi-coated Some surfaces, but not all, are multi-coated.
- Fully multi-coated All air-to-glass surfaces are multi-coated.
The amount of light that reaches your eyes after entering the binoculars (measured as a percentage).
How many times larger an image will appear compared with that seen by the naked eye.
Objective lens diameter
The measurement of the lenses at the end of the binoculars where light enters them (measured in millimetres). The larger the measurement, the more light the binoculars can take in, making the image appear brighter (all other factors being equal). Generally, binoculars with an objective lens greater than 30mm are classed as standard-sized binoculars. Those with an objective lens diameter less than 30mm are classified as compact binoculars.