The exit pupil is located just before the eyepieces and ideally matches the diameter of the eye pupil of the user. Formula for calculating the diameter: Lens diameter : Magnification number. The larger the diameter of the exit pupil, the better the viewing result at dusk, since the human eye pupil widens at low light levels (approximately 7 mm for young people and approximately 5 mm for those of the age of 50). During the day, a diameter of approximately 2 to 4 mm is sufficient. However, the exit pupil says nothing about the quality of binoculars.