Wikipedia
The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece at which the user's eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained. The calculation of eye relief is complex, though generally, the higher the magnification and the larger the intended field-of-view, the shorter the eye relief.
Usage examples of "eye relief".
Some are physical size and eye reliefthe distance between the eye of the firer and the rear sight.
That permitted me to follow flying birds, but birds are so close in Belize that often I could only use my glasses by increasing the eye relief.
The eye relief was so accurate that the recoil would knock the eyepiece back to within one millimeter of Stephen's eyebrow and yet never touch a hair.
The eye relief was so accurate that the recoil would knock the eyepiece back to within one millimeter of Stephen’.
Ling had a scope--a monocular whose eye relief allowed it to be used through his face plate--but this was little help.