The circular image or beam of light formed by the eyepiece of a
telescope. To take full advantage of a scope's light-gathering capacity,
the diameter of an eyepiece exit pupil should be no larger than the 7mm
diameter of your eye's dark-adapted pupil, so that all of the light
collected by the telescope enters your eye. (The eyepiece exit pupil
diameter is found by dividing the eyepiece focal length by the telescope
focal ratio.) Your eye's ability to dilate declines with increasing age
(to a dark-adapted pupil of about 5mm by age 50 or so). For those in
this age group, eyepieces with exit pupils larger than their eyes can
dilate to simply waste their telescope's light-gathering capacity, as
some of the scope's light will fall on their iris instead of entering
their eye.