Eye relief is a term that refers to the position of a rifle's scope relative to the position of the shooter. Specifically, eye relief is the distance from the back of the scope to the shooter's sighting eye, generally measured in inches. Rifles vary in terms of eye relief. Scopes vary depending on the power of magnification and other factors.
How Eye Relief Works
Eye relief is not determined by how far ahead the shooter likes to place his face, nor by how far back on the gun the scope is installed. Eye relief is the distance at which you can see a full view through the scope. If you move your eye further back from the lens, the field of view begins to constrict. The visible circle of the target area becomes smaller and more wobbly, due to parallax. Only at the precise distance for which the scope is calibrated will the entire field of view appear.
Eye Relief of Various Calibers
Generally, the more powerful the scope, the lower the eye relief. Most high-powered hunting rifles do not come with the highest-magnification scopes, but are rather equipped with variable mag scopes in the range of 3X-9X or 3.5X-10X. These scopes have an eye relief of three to four inches, which is enough to adjust for the recoil of the larger rounds.
Small Eye Relief
If eye relief is too small, it can be a hazard for the shooter. The recoil of the weapon can jolt the scope back into the shooter's eye or forehead. The metal edges of hunting scopes are sharp enough to cut through skin, and many a shooter has required stitches after a recoil bounced the scope off his head. Some scopes are equipped with rubber edging to avoid cutting, but needless to say, the impact is still far from pleasant. All shooters should be wary of insufficient eye relief when choosing a scope.
Large Eye Relief
A large eye relief spares the shooter the risk of injury, and also can make it easier for the shooter to locate the target. While scopes with a short eye relief require careful positioning of the shooter's head, scopes with larger eye relief offer a range of positions. The shooter only has to get his eye into a range of distances, far enough from the scope to be safe and close enough to be within the eye relief distance.
Variable Eye Relief
Eye relief changes on most hunting rifle scopes. Most scopes are variable magnification, and eye relief depends on magnification. When the scope is adjusted for higher magnification, the eye relief will decrease; when it is adjusted for lower magnification, the eye relief increases.
Mixing & Matching Scopes
Dangers arise when shooters choose a scope with more magnification than they need. Most higher-magnification scopes are for varmint guns, which shoot smaller rounds in the range of .22 and .223 centerfire. These rounds do not have a strong recoil, making the small eye relief safe. But when these scopes are mounted on more powerful rifles, in the class of .30-06 or .270, the small eye relief and larger recoil can be dangerous.