What are "exit pupils" on a pair of binoculars?

Q. What are "exit pupils" on a pair of binoculars?

A. The exit pupils are the apertures that you look directly through as you put a pair of binoculars to your eyes. All other things being equal, the larger the diameter of the exit pupil the more light your eyes will receive.

You can calculate the size of an exit pupil by dividing the objective lens size by the magnification of the binoculars. Using this calculation you can tell that a pair of 10x32 binoculars will have an exit pupil of 3.2mm, and 8x42 binoculars will have an exit pupil of 5.25mm.