This term is a measure of distance to the point behind the eyepiece where you must position your eye so you can see the entire field of view. Short focal length eyepieces normally have shorter eye reliefs. However, for a person who must wear glasses, this prevents them from positioning their eye sufficiently close to the eyepiece. To solve this problem, some eyepieces are now designed with a longer eye relief and include a rubber eyeguard to shut out stray light and help those who don’t wear glasses to position their eyes.