Scope Eye — Optical v. Functional Eye Relief

By Ron Spomer published on in How To, Hunting, Safety and Training

Eye relief can be more than drops in a tiny bottle. It can be a critical part of riflescope performance. If you hope to fire a powerful rifle without cutting your eyebrow with the scope, you need to understand and employ proper eye relief (ER.) There are two kinds: optical and functional.

Woman holding a bandage and bleeding from the forehead

Minor injuries such as “Scope Eye” aren’t necessarily life-threatening, but the right gear in your pack, including derma-bond, bandages, and analgesics can certainly make them more comfortable.

Optical ER is the distance from the external surface of a riflescope’s eyepiece glass to the outside surface of the shooter’s eye at which the full image transmitted by the scope is visible. In other words, you can see everything without those black, shadowy edges. When you see those, you are either too close or too far back from the eyepiece. Go ahead. Try it. Move your eye close to your scope and watch the shadowy edges close in.

Next, pull back and watch them disappear. Keep pulling back and watch them reappear. You are moving in and out of the scope’s preset ER distance, which can be anything from two inches to two feet. Optical ER is something you cannot adjust. Each scope comes with a preset ER and you have to live with it, or live without that scope.

Functional ER is measured from the rear-most rim of the eyepiece barrel to your brow when you can see a complete a full view through the scope. If you have deep-set eyes and your scope has a deeply set eyepiece lens, functional eye relief can be as much as an inch less than optical eye relief. This is a major contributor to painful “scope eye.” If optical ER is too long, you sacrifice field-of-view. If functional ER is too short, you’ll probably get bashed in the forehead each time you fire a shot.

Wide-angle scopes achieve their wider view by sacrificing a bit of ER. This is acceptable on a light-recoiling rifle, but a hard kicker increases your odds for stopping the scope with your forehead. I’ll gladly sacrifice two, or even ten feet, of FOV for another inch of ER in most scopes because I don’t need to see half a football field around my target animal. I need to focus on the animal—without getting bonked on the brow by my scope when I shoot.

As a general rule, the harder kicking your gun is, the longer you want your scope’s ER. A mere two inches of ER might be plenty atop a .223 Remington, but 3.75-inches might not be enough atop a .30-378 Weatherby Magnum. Most manufacturers build riflescopes with 2.75 to 3.5 inches of optical ER, some as much as 5.3 inches. About right for most common deer and elk cartridges from .270 Winchester through .300 Win. Mag. are 3.25 inches and longer, but take functional ER into account. Things get a little sticky with bigger .300s into the .375 H&H. From .375 on up through the .505 Gibbs, you want at least 4 inches of ER—if you want a scope sight at all.

Flashlight beam being shone through a riflescope to measure eye relief

Shining a flashlight through a scope focuses the circle of light when the paper or wall is at the optical eye relief distance of that scope. Measure from the wall to the edge of the eyepiece barrel to get the functional ER distance.

For obvious reasons, handgun and scout rifle scopes are built with extra-long ER of 10 to 18 inches. To get this they sacrifice field-of-view. That’s why looking through one is like peering down a long tunnel. Even with standard riflescopes you trade field-of-view for ER. The farther your eye is from the scope’s eyepiece, the narrower your downrange view. It’s like peeking through a small knothole. The closer you get to the hole, the more area you see beyond it.

The way you shoot also matters. Stock “creeping” contributes to scope eye. If you get excited while hunting and slide your head farther forward on the comb than you normally do when practicing, you could easily shorten functional ER by an inch. Prepare to bleed. Shooting prone and uphill and you’ll bring your brow dangerously close to your scope eyepiece rim, too. At the same time your butt pad will tend to slide under your shoulder pocket. It’s the perfect set up for a scope gash.

Determine Your Scope’s Eye Relief

Finally, be aware that many, if not most, variable power scopes lose ER as power increases. Watch out for this when buying. Be aware that some manufacturers fudge their ER numbers. You can double check them with a flashlight and sheet of paper. To measure ER, lay a ruler in front of a scope’s eyepiece atop a sheet of white paper on a table. Put a right angle bend in the paper. Now put a flashlight against the objective bell of the scope. Shift the scope back and forth over the ruler until you see the sharpest circle of light on the paper. That’s the sharpest focus and the true optical eye relief. Measure from the eyepiece rim to the paper to determine functional eye relief.

Have you ever suffered from “scope eye?” Tell your story in the comment section.

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Veteran outdoorsman Ron Spomer began writing and photographing about wildlife, hunting, guns, optics and all things wild and wonderful waaaay back in 1975. He’s been privileged to have hunted on six continents for small game, upland birds, waterfowl, big game mammals, and — no croc — even some reptiles. From the Arctic to the equator, from mountain tops to ocean marshes, Spomer celebrates our hunter-gatherer heritage. You can see him on his You Tube channel and follow him onRonSpomerOutdoors.com.

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Comments (15)

  • Mark

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    I have technical questions about how to check for ER. Do you set the scope on max magnification (I have a 3.5 X 10 scope). When shinning the light into the scope there are a least 2 rings shown on the paper. Which ring (Inner or Outer) is the one that you need to check for sharpness. Also, I noticed moving the flash light forward and backward will change the image too. Where should the flashlight be positioned ?.

    Reply

  • John Anderson

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    I contracted a nasty case of scope eye back in 2006 or 2007. I mounted a nice little Leupold VX-3 1.5 – 5 x 20mm scope on my shiny new CZ 550 Safari Magnum with European stock, in .458 WinMag. I had the rather silly notion of using it on a bison hunt. The second shot I fired from the box of Haornady Light Magnum 500 grain soft points whapped me good, right in front of lots of my friends. The scar healed rather nicely, never quite got the blood stains out of that tee shirt though. Needless to say I worked up some handloads for this rifle, although I must say, this rifle is NOT my favorite rifle to shoot off the bench.

    I ended up taking my bison with a Remington 700 Classic in 8mm Mauser with 180 grain Barnes TSX all copper handloads. I couldn’t get close enough on the opening shot to be confident of hitting the animal in a vital spot, as these critters in this herd were pretty wary.

    Absent the need for a hunting rifle with real stopping power, my big .458 is now mostly a curiosity that I toy with from time to time. I’m thinking of taking the scope off of it since I can probably do as good or better with open sights at any range I care to take a shot at something with this gun.

    Live and learn, and have fun doing it; we only get to do this “life” thing once!

    Reply

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