Male American Kestrel

During a recent birding walk with a large group of people, I offered my binocular to another birder so she could get a better look at the American Kestrel perched on a branch ahead of us. Her binocular (probably a family heirloom from her grandfather) didn’t allow her to see the blue-gray wings of this male. My binocular didn’t help though; in fact, she couldn’t really see anything with mine. Then I saw the problem: She was wearing thick eyeglasses, and I don’t use glasses in the field. Simple fix, we had an eye relief issue!

Eye relief is the ideal distance between your eye and the eyepiece of your binocular or spotting scope. It’s when you get a full view with no black ring around the edge of your view. If your eye is too far from the eyepiece, you limit the field of view, diminishing your birding experience. This is particularly important if you wear eyeglasses because the lens of your glasses increases the distance between your eye and the binocular.

Eyeglass wearers need longer eye relief to take full advantage of binocular optics. Typically, the desired long eye relief is 16 mm or more, but this can vary. The specifications on a binocular should list eye relief, but it’s best to test an individual binocular with your the glasses on, otherwise you may not get the best views.

Most modern binoculars have adjustable eyecups that twist up and down, allowing the user to adjust eye relief. Don’t overlook the importance of quality in adjustable eyecups. On better binoculars, when you twist the eyecups they lock at set points to help ensure the eye relief is equal for both eyes. Locking also means the eyecups are less likely to shift by accident.

On some models there is sufficient friction in the eyecup motion to achieve the same end. If one eyecup moves without your knowledge, your next view through the binocular could be cockeyed, costing you the opportunity to focus on a fast-moving bird. A smooth twisting action in eyecups is one sign of a quality binocular.