Binoculars:
Eye Relief
by Michael and Diane Porter
People who wear glasses sometimes get cheated out of part of the image they should be seeing through a binocular. If the eye relief is too short, they see only the center of the picture. Like this:
The field will be constricted if your glasses hold the binocular’s eyepiece too far away from your eyes. That is what happens if the binoculars don't have long enough eye relief.
But a binocular with long eye relief shows you the full image, whether you wear glasses or not. Like that:
Eye relief defined
It's all in the design of the eyepiece. For any binocular, there is an ideal distance that your eye should be from the eyepiece. This distance is called the eye relief.
If your eye is farther away from the eyepiece than the eye relief distance, you lose the outer edge of the picture. And the farther away your eye is, the smaller portion of the picture you see.
So you want a binocular whose eye relief is long enough to accommodate your eyeglasses. Otherwise, it's like paying for a
box seat but watching the game through a hole in the fence.
A binocular with long eye relief is also OK for a person who does not wear glasses, because the eye cups extend to hold the eyes the correct distance from the eyepiece.
What is a good eye relief for glasses wearers?
 If you wear glasses, you need binoculars whose
eyepieces are specially designed with long eye relief. You need a minimum of about 16mm or longer.
Many binoculars have eye relief of only 9 to 13 mm. Even though the eyecups fold down to let glasses wearers get closer, it's not close enough. Such binoculars are suitable only for people who don't wear glasses (or who use contact lenses).
Exactly how much eye relief you need depends on your glasses. If they are small and close to your eyes (like John Lennon's), you might get away with a binocular whose eye relief is only 14mm. (Most glasses wearers need longer eye relief than that.)
Manufacturers publish the eye relief in the specs of each binocular, but you can't always rely entirely on the numbers. There are different ways of measuring the eye relief, with inconsistent results.
 Zeiss, for example, lists 16mm eye relief for their Victory FL 8x42, while Pentax lists 22mm on their DCF SP 8x43. Yet, in our tests at Birdwatching Dot Com, we found the Zeiss to have at least as generous eye relief as the Pentax.
Although the published eye relief can help you select a binocular, we recommend that if possible you actually try a binocular you're considering buying. Try it with your glasses on and with your glasses off, and notice whether the image is just as wide with your glasses as without them.
If your binocular's
eye relief is not sufficient for your eyeglasses, you're missing the big
picture, a lot of the pleasure, and possibly the bird.
— © 2008 Michael and Diane Porter
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