The dusk number is important if you use the binocular especially in the dark or in poor lighting conditions. The higher this number, the better the viewer performs in low light. A dusk number of less than 15 is actually only suitable for daytime. The dusk number is calculated by multiplying the diameter of the lens with the magnification factor and extracting the root from this number.
Dusk number = √ (diameter lens x magnification factor)
Which dusk number is best differs per person and per situation. If you use the binoculars in the Netherlands, where there is a relatively long dusk time, a binocular with a high dusk number will provide a good result. For example, if you go to the tropics, where there is a very short dusk time, brightness and light intensity are very important.