Eye Relief in Binoculars
People who wear glasses sometimes get cheated out of part of the image. If the binocular's eye relief is too short, they see only the center of the picture. All that good stuff around the edges is obscured.
The thing is, every binocular is made so that there is an ideal distance from your eye to the glass of the eyepiece. That distance is called eye relief. Each binocular has a particular eye relief, depending on the optical design. Each binocular has a certain eye relief. It's usually between 10mm and 20mm. In order to see the whole picture, you need to locate your eye at that distance from the eyepiece of the binocular.
If your eye is too close, you get weird shadows coming in from the sides. If you eye is too far away, the image will be constricted.
If you wear glasses, the eyeglasses keep your eyes farther away from the glass. But you still have to get your eyes at the right distance. If your glasses won't let you get your eye get close enough, you'll lose the outer part of the image, and that nearby Great Blue Heron won't fill your field of view. Instead you'll see only the center of the image.
The farther away your eye is, the smaller portion of the picture you see. It's like you've paid for a box seat but ended up watching the game through a hole in the fence.
Long eye relief
Binoculars come with eyecups that you can pull or turn to make the eye relief shorter or longer, to work with your eyes and glasses. But if you wear glasses, you need binoculars with long eye relief. Room for your glasses to fit in there without keeping your eyes too far from the binocular's eyepieces.
What is good eye relief?
Most glasses wearers need binoculars with a minimum of about 16mm of eye relief. However, exactly how much eye relief you need depends on your glasses and your face. If your glasses are small and they ride close to your eyes, you might get away with a binocular whose eye relief is only 15mm. Most glasses wearers, however, need longer eye relief than that.
How to determine eye relief
Try the binocular with your glasses. Look through it with your glasses on and with your glasses off. Never mind if you can't focus without your glasses. What you're trying to determine is whether the image includes everything with your glasses on that you can see with your glasses off.
Manufacturer publish the eye relief in the specifications of each binocular, but you can't always rely on the numbers absolutely. There are slightly different ways of measuring eye relief, with differing results. If you're not sure the binocular has long enough eye relief for your glasses, it's best to try it in person. Or select a model with generous eye relief, such as 18 mm.
For non-glasses wearers?
And note that you do not wear glasses, none of this matters to you at all. Binocular with long or short eye relief will work just fine for a person who does not wear glasses. If the binocular has long eye relief, the eye cups can be extended to hold the eyes the correct distance from the eyepiece.