A Primer In Optics.

The exit pupil is the diameter of the light beam that leaves an optical system's eyepiece and enters the eye. It's a virtual aperture in the optical system, and only rays that pass through it can exit the system. 

Here are some things to know about exit pupils:

Binoculars

The exit pupil is the bright circle in the center of each eyepiece. It's calculated by dividing the objective lens size by the magnification power. A larger exit pupil means a brighter image, which is important for low-light conditions. 

Telescopes

The exit pupil of a telescope should correspond with the amount of dilation of your eye's pupil after it's fully dark-adapted. If the exit pupil is too large to fit into your eye, you lose some of the instrument's incoming light. 

Factors that affect exit pupil

The more an eyepiece magnifies, the smaller the exit pupil. The smaller the focal length, the smaller the exit pupil. 

The bright dot in the eyepiece is the exit pupil.

Measuring my own pupil.

Different exit pupil sizes for different observations.

A 2mm exit pupil is typically best for planetary, lunar, and globular cluster observing. 

A 1mm exit pupil is excellent for splitting binary stars. 

A 0.5mm exit pupil is useful for splitting close double stars, but only during very good seeing.