Exit pupil

Exit pupil is a way of measuring the size of the light beam that comes out of the eyepiece of a telescope or binoculars. It is measured in millimeters (mm). The bigger the exit pupil number, the more light comes out of the eyepiece and the easier it is to see stars and other objects in the night sky. To measure the exit pupil, take the diameter (width) of the eyepiece lens and divide it by the magnification number of the telescope or binoculars. For example, if the eyepiece has a diameter of 24mm and the magnification is 10x, then the exit pupil is 24 divided by 10, which is 2.4mm.