Eye relief is the distance between the eyepiece lens of a rifle scope and the shooters eye that allows for a full field of view without obstruction. It is an important factor in selecting a scope for a specific rifle and purpose. Eye relief is a specific, set, inherent phenomenon to that specific optic, and there is no changing the eye relief, only the shooter’s position or the scopes position on the rifle.

Proper eye relief is essential for a successful and enjoyable day at the range. If the shooters eye is too far back, the field of view will narrow and give a tunnel vision effect, while if the shooters eye is too close, the picture can be distorted as well. Eye relief is also important for safety reasons. If the scope is mounted in such a way that the shooter has to hunch up close for a clear field of view, the recoil can cause the scope to smack into the shooters eye, resulting in a condition known as "scope eye".

The industry average for a fixed-power scope is about 3 1/2 inches, while for most variables, the eye relief starts out at about 3 1/2 inches at the lower power and about 2 1/2 inches when cranked up to max magnification. Handgun mounted scopes require long eye relief, which is measured in feet instead of inches due to the necessary length. The amount of eye relief needed depends on the type of firearm and the shooters posture and fit.