The exit pupil is the circle of light you see when holding a binocular or spotting scope about 6 inches away from your eyes and looking at the surface of the eye pieces. This little bright circle is the amount of light reaching your eyes.
The size of the exit pupil is easy to calculate. You simply divide the magnification of the binocular into the diameter of the "objective lens" or bottom lens of the binocular or spotting scope. So, a 7x35 binocular has an exit pupil diameter of 5mm (35 divided by 7). A 10x50 binocular also has an exit pupil of 5mm (50 divided by 10).
