The distance between the rear lens (the eye lens) of an eyepiece and the exit pupil. If this distance is too short, the eye will be uncomfortably close to the eye lens; if it is too long, it may be hard to keep the eye in the best position to view the image. In general, eye relief gets shorter as the focal length of the eyepiece decreases, but it is possible to design eyepieces of short focal length with good eye relief. Good eye relief is useful for spectacle wearers.
Subjects: Astronomy and Astrophysics.