Eye relief is more technical than you probably think.

We all know what eye relief is, right? It's the distance between the eye and the lens that gives us a full-diameter field of view through the scope, without tunneling or blacking out at the slightest movement. We also know that eye relief is different for different types of scopes and different magnifications.

Generally speaking, general-purpose riflescopes have eye relief in the three- to four-inch range. Scopes designed for compact short-magnum rifles and cantilever-mounted shotguns and slug guns have longer eye relief (as much as five or six inches), and scopes designed to be mounted on barrels (scout rifles) or handguns have extremely long eye relief for both-eyes-open shooting (and can usually be used interchangeably).

In optics catalogs or specification sheets, eye relief is usually given as a single measurement for fixed-power scopes, and as two measurements for variable-power scopes--one measurement for the lowest-power setting (longer), and one measurement for the highest-power setting (shorter). These figures give the impression that optimum eye-relief occurs only at one specific spot for each different magnification.

In reality, the distance behind the scope where your eye can see that full-diameter field of view is a zone, not a spot. Some manufacturers refer to this zone as the eyebox, meaning the area between the closest position and the farthest position from eye to lens where you can still see the full-diameter view.

When preparing for your next hunt, give more thought to the position of your scope--and neck; your forehead will thank you.

In general, for scopes of similar design specifications, lower magnification scopes have deeper eyeboxes than high-magnification scopes. Obviously, a longer eyebox is better than a shorter eyebox, because it gives you more flexibility in your head position on the stock, and more flexibility in the exact forward/backward mounting position of your scope on your gun.

It's also important to remember that with variable-power scopes, the eyebox is not constant to all magnifications; it moves forward or backward as magnification is raised or lowered. So if a scopemaker's catalog tells you the eye relief on its 4.5-14x40mm magnum big-game scope is 4.4 inches at 4.5X magnification and 3.6 inches at 14X magnification, the reality may be that 4.4 inches is the center of a 1.5-inch deep eyebox at 4.5X, and 3.6 inches is the center of a 1.0-inch deep eyebox at 14X. That means that when your eye is positioned from 3.75 inches to 4.1 inches behind the lens, with this particular scope, you would have a full-circle field of view both at 14X and 4.5X and every magnification in between, because that's within the overlap of the eyebox range for both extremes.

Therefore, when you mount this scope, you would want to position it to be about 3.9 inches (give or take a freckle) in front of your eye with your head in normal shooting position on the stock, so you will not have to shift position to have a full field of view anywhere throughout the entire magnification range.

Unfortunately, not all catalogs report eye-relief specifications the same. In some scopemakers' catalogs, the eye-relief specification means the minimum distance you can see a full field. In other catalogs it means the maximum distance you can see a full field. In still others (the majority, fortunately) it means the middle of the eyebox. But nobody tells you how deep their eyeboxes actually are. With any scope out there, the only real way to find out what the eye-relief range is is to measure it yourself on your own gun.