Eye relief is the distance between the eyepiece of a viewfinder and the photographer's eye at which the whole viewfinder image can still be seen. A longer eye relief lets you hold your eye farther back and still see the full frame.
This matters most for people who wear glasses, because the glasses keep the eye away from the eyepiece. Camera makers usually list eye relief in millimeters. Some viewfinders also have dioptric correction so people can use them without glasses.
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