Exit Pupil

The diameter, in millimeters, of the beam of light that leaves the eyepiece of a pair of binoculars is the "exit pupil". The larger the exit pupil, the brighter the image obtained will be. Having a large exit pupil is advantageous under low light conditions and at night. For astronomical, marine or low light applications, the exit pupil of the binocular should correspond with the amount of dilation of your eyes pupil after it has adapted to the dark. This number will be between 5mm and approximately 7.5 mm. 7 to 8mm of dilation is normally the maximum amount for the human eye, and this number tends to decrease with age.

To calculate the exit pupil, divide the size of the objective lens by the magnification of the binocular. For example, the exit pupil of 7x50 binoculars is 50 รท 7 = 7.14mm.