Waldberg 10 x 42 rangefinder binoculars

Calculation of the twilight index: square root of the product of magnification and diameter. For 8x42 binoculars, this is the root of 8x42, that is, the root of 336 = 18.3.
The higher the crepuscular index, the better. Indeed, the higher this index is, the more we will see details. There is another criterion for binocular clarity: brightness.
The brightness is obtained by squaring the diameter of the exit pupil (42/8 = 5.25 mm), that is to say by multiplying it by itself. In our example, the relative brightness is: 5.25 x 5.25 = 27.6. The higher this value, the better. When less than 15, binoculars are especially suitable for daytime use.
Binoculars with a twilight index greater than 25 may be called night binoculars. The limit is however fuzzy. A high crepuscular index combined with low brightness (12x40 binoculars have a twilight index of 21.9 but a brightness of 11.1) will be difficult to handle.