What eye relief means for shooters
The term "eye relief" commonly refers to the distance between your eye and the eyepiece lens of an optical instrument when your eye is positioned so that you can see the entire field of view. In recreational shooting, the optical device is most likely a scope, spotting scope, binocular, or rangefinder. Although the concept of eye relief is simple enough, we can find ourselves in a bind if we treat it like Rodney Dangerfield and don't give it proper respect.
An older spotting scope with limited eye relief compels user to mash eyeglass lens against eyepiece for a full view.
A serious pitfall with eye relief stems from taking the conventional definition too literally. For shooters, eye relief should really be redefined as the distance between your eyeglass lens and the rearmost projection of the eyepiece when you can see the entire field of view. That's a whole 'nother animal because it is a much shorter distance than the cornea-to-lens distance of the classic definition, which means that we will need more eye relief than the conventional measurement might allow. (Our modified definition of eye relief assumes that no prudent shooter would risk eye injury by using a firearm or being near active shooters without protective eyewear.)
When choosing sporting optics--in particular, riflescopes--carefully reading specifications can help eliminate models with scant eye relief. As a rough guide, a riflescope that provides less than 3 inches of eye relief might best be used on a rimfire rifle or a centerfire that generates minimal recoil. If you're addicted to magnums or if you tend to stock-crawl with a moderate kicker, take out some eye insurance in the form of a scope that provides eye relief from 3.5 to 5 inches, which will help avoid a too-close encounter with a recoil-driven eyepiece. The extended safety zone will be worth the search, particularly when the day comes (remember Murphy's Law) that you whip the rifle up fast for a fleeting shot or contort into an unaccustomed shooting position. Similar considerations apply when scoping handguns; let recoil be your guide.
Spotting scopes and binoculars are sometimes stingy with eye relief, presenting the eyeglass-wearer with what looks like the view through a soda straw. Increasingly, manufacturers have seen the light and offer instruments with long-eye-relief optics that allow us four-eyed users to see the full field without ever clicking a spectacle lens against the eyepiece rim. There is a practical benefit here that goes beyond mere comfort and convenience. If your specs don't touch the eyepiece, the lenses don't collect scratches. Prescription eyeglass lenses aren't cheap.
An Aimpoint red-dot sight on this .45 ACP bullseye pistol offers extended eye relief appropriate to arm's-length NRA offhand stance, recoil level, and flying brass.
When you check the eye relief of any sporting optics prior to purchase, be sure to do so while wearing the same eyeglasses you expect to wear while using the instrument out in the real world. Different eyeglass frame styles can position lenses at different distances from the eye, and thus from the eyepiece(s) of a riflescope, spotting scope, or binocular. Your shooting glasses and around-town specs may be configured quite differently and might even have a somewhat different prescription.
My own prescription for assessing eye relief is to think of it as optical chocolate: You can rarely have too much. As Martha Stewart might observe, "It's a good thing."