Eye relief is the distance between the rear lens (eyepiece) of the optic and your eye at which you see a full, clear field of view.

For rifle applications, inadequate eye relief means the scope can strike your eye under recoil — the “scope bite” that cuts the eyebrow or forehead. This is a safety issue, not just a comfort issue.

Night vision scopes typically require longer eye relief than comparable daytime scopes because the user may be wearing a helmet, face protection, or other head-mounted equipment. Look for devices with at least 40–50mm of eye relief for any rifle application.

For handheld use (monoculars, binoculars), eye relief affects comfort and field of view but is not a safety concern. Eyeglass wearers should look for “long eye relief” models that maintain full field of view with glasses.