Binocular Guide - Lenses and Prisms

The most important parts of a binocular are the lenses and prisms, the quality of these being the main factor in how binoculars perform and the price you pay.

Prisms: The quality of prism is defined by its 'BaK' number. There are two different numbers, BaK7 and BaK4 - BaK4 being the better of the two. These ratings do not fully show how good the prisms are, a good BaK7 prism can be better than a poor BaK4 prism. Having said that they are still a very good indication of prism quality. A good BaK4 prism will create a very bright, crisp image.

Lenses: The quality of lens and its coating also affects the image. Some more expensive binoculars have Aspheric Lenses which are specially moulded for superb edge definition. A good lens and coating reduces the reflection and thus increases brightness and crispness. Lens coatings can come in different colours, the most common are:

• Blue - A very common coating. This gives a neutral colour balance and a clear bright image,
• Green - This helps to increase the contrast in the image and to improve colour rendition,
• Red - This is an anti-UV coating. It cuts out the Ultra-Violet part of the spectrum and so makes the image clearer on bright hazy days. It can affect the colour clarity, turning the image slightly blue. It is not for seeing in the dark.
• Gold - This is an anti-UV coating. It cuts out the Ultra-Violet part of the spectrum and so makes the image clearer on bright hazy days, but without altering the colour balance significantly.

The more glass surfaces within a pair of binoculars which have coatings, the less light will be wasted and scattered by reflection, so the image can be brighter. More expensive binoculars can be fully multi-coated: all surfaces have various coatings.