Eye relief is the distance from the surface of the eye piece to your eye where you are using the optic to its fullest potential.Outside of the eye relief distance you have a degraded field of view, meaning you aren't getting the whole picture.
Long eye relief is desirable for a few reasons on a rifle:
- The optic may be mounted further forward on the rifle. This allows for a back up iron site to be easily mounted behind the optic and the magnifier.
- Eye protection can cause your eye to be further away from the optic. This allows for plenty of room to maintain a consistent (and comfortable) cheek to stock weld.
- More distance between the shooter and the optic also allows room for recoil and can eliminate getting knocked in the face with the optic.