Binoculars are characterised by two numbers e.g. 8x42. Both are important. The first number is the magnification: how many times the object viewed is magnified. It does not matter whether it is a bird in a tree or the Moon, both objects are magnified the same amount. The second figure is the size, in millimetres, of the diameter of the objective lens. For the same given magnification, the larger the size of the objective lens, the more light can pass through the binocular into your eye and therefore the brighter the image. Of course, a larger objective lens means a larger and therefore heavier binocular.
To determine the brightness of the image, we divide the objective lens size by the magnification. This number, in millimetres, gives us what is known as the exit pupil. If you hold the binocular at arms length and view the binocular through the eyepiece, you will see a small round circle of light. This is the exit pupil. So, 8x42 and 10x42 binoculars give an exit pupil of 5.25 and 4.2 mm respectively. For this reason, an 8x42 binocular may be the preferred choice if you are going to use the binocular in poor light conditions. Similarly, an 10x25 binocular will provide an image that is less bright than its bigger cousins. However, this size of binocular, at approximately half the weight may be a good choice for walkers or holiday makers for example.
The field of view is how wide the angle of view that the binocular yields. Generally, an 8x magnification is wider than a 10x which makes an 8x a good choice when viewing near of fast moving objects. It becomes less important for long distance observation where, even if the object is moving, the relative speed is small.
Field of View can be measured either by degrees or a measure of width at a given distance e.g. 119m at 1000m (1km). In this case, if viewing an object 1km away, the distance between the edges of the image are 119m apart.
The type of lens coatings is very important in determining the quality of the image viewed.
• Fully Coated: Here, there is at least one coating on both sides of both the objective lens and the eye piece lens system.
• Multicoated: Multiple layers of coatings on one or more of the lens surface.
• Fully Multicoated: Multiple coatings on all the lens surface. These are high quality optics
• Phase Coatings: Only applies to roof prism binoculars. This coating reduces the polarisation of light as it passes through the prism and greatly improved brightness and sharpness. The Vivid HD and MX2 series have phase coated prisms.