Binoculars glossary Twilight factor

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The 'twilight factor' can be calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result. For instance, the twilight factor of 8x42 binoculars is therefore the square root of 8x42: the square root of 336=18.3. The higher the twilight factor, the better.

In fact, the higher the twilight factor, the better the resolution of the binoculars when observing under dim light conditions. There is also another number that indicates the level of clarity and brightness in binoculars. Brightness is calculated by squaring (i.e. multiplying the figure by itself) the diameter of the exit pupil (42/8=5.25 mm). In our example, this means that the relative brightness index is 27.6 (5.25x5.25=27.6).

The higher the number, the better. Any value under 15 indicates that the binoculars are mainly suitable for use during daylight hours. Binoculars with a relative brightness index above 25 fall into the category of night vision binoculars. However, these parameters are obviously flexible. A high twilight factor coupled with a low relative brightness index (12x40 binoculars have a twilight factor of 21.9 but a relative brightness index of 11.1) will still be difficult to use effectively.

In a country such as the Netherlands with a relatively long period of dusk or twilight, it is often better to choose binoculars with a high twilight factor. However, in the tropics, for instance, dusk or twilight is of a very short duration and therefore the relative brightness index is more important. (See also: Exit pupil).

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