What is the exit pupil for binoculars?

The exit pupil is the width of the beam of light leaving the eyepiece, usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of the binocular should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5 mm and 9 mm (the maximum amount for the human eye). Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam falls on your iris and doesn't go into your eye. To calculate the exit pupil, simply divide the size of the objective lens by the magnification of the binocular. For example, the exit pupil of 7x42 binoculars is 6 mm.

Updated 10/23/13