Why are some binoculars brighter than others?

One of the most important factors affecting the image brightness is known as Exit Pupil, which is a bright circle visible when the eyepiece array is viewed about 10 inches away from the eyes. A larger Exit Pupil gives a brighter image. The value of Exit Pupil (mm) = Objective Lens Diameter / Magnification Power. So, for a pair of 10x40 binoculars, its Exit Pupil is 40/10=4mm. For the Nipon 10x50 binoculars, the Exit Pupil is 5mm. The larger the objective lens, the bigger the Exit Pupil, and thus the brighter the image viewed. However, larger objective lens also means heavier and bigger the binoculars' body.

Binoculars for common use such as sightseeing and birding from near to medium range, an exit pupil size of 4-5mm is considered to be adequate for image brightness.