exit pupil

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.