Just as light enters the binoculars, a certain amount of light will understandably exit at the other end. How much light exits the binoculars depends on its aperture and magnification. The beam of light from the eyepiece enters the eye, and is converted into a decipherable image with the help of the brain.
The glasses exit pupil is an important factor for observing purposes. But first of all let’s take a look at the human eye: the pupil has a diameter of around 5-8 mm. At night, when it is dark, the pupil opens the widest, so that as much light as possible can enter the eye. In younger people, the pupil can dilate by up to 8mm at night. In older people the pupil’s maximum dilation is smaller.
Info: The exit pupil (AP) refers to the width of the beam of light which exits the binoculars or eyepiece. This means that the exit pupil should not be greater than the aperture of the eye's pupil (or ideally equal to EP, the entrance pupil of the eye), otherwise light is lost.