Glossary
Exit pupil
The width of the light beam leaving an eyepiece, found by dividing aperture by magnification, which determines how bright an image looks.
Exit pupil is the diameter of the narrow beam of light that leaves an eyepiece and enters the observer's eye, calculated by dividing a telescope's aperture by the magnification being used. It is measured in millimeters, and it directly determines how bright an image appears, since a wider exit pupil delivers more of the telescope's gathered light into the eye at once for a given object.
A human eye's pupil dilates to roughly five to seven millimeters in full darkness, depending on age, so once a telescope and eyepiece combination produces an exit pupil larger than the observer's own dilated pupil, the extra light is simply wasted rather than making the image any brighter. This is why very low magnification on a large-aperture telescope does not keep getting brighter indefinitely, and why exit pupil is a useful sanity check when choosing eyepieces.
At the other end, a very small exit pupil, well under one millimeter, produced by high magnification, delivers a dim, difficult-to-focus image even on a bright object, which is one of the reasons pushing magnification too high on faint targets backfires. When choosing eyepieces for a given telescope, checking the resulting exit pupil at each magnification helps avoid combinations that will look either strangely dim or no brighter than a lower-power option.