The Exit Pupil of a telescope (or binoculars or microscope or
other optical device designed for personal viewing)
is the diameter of the disk-shaped region where
your eye should be to perceive the intended image.
This region is some distance from the device's Eye Piece
(the distance known as the device's Eye Relief),
so that despite the physical characteristics of the
instrument, the eye lens can be brought to the region
indicated by the exit pupil. The exit pupil is
best sized to match the diameter pupil of the user's eye (the
user's Pupil Size),
which is at its maximum with night vision and is larger
for young adults than older adults. If an ill-designed instrument
does not allow your eye lens to line up with the exit pupil, parts
of the image can be blurred.