For every pair of binoculars, there is an optimal distance between your eye and the eyepiece, which is called eye relief. If your eyes are too close to or too far from the eyepieces, you can’t see the whole picture: part of it is blacked out. This is called “vignetting“.You do not want to be as close to an elephant as I was (4/10/2010, Lower Sabie, Kruger National Park, South Africa) and not have the full field of view available!! If your eyeglasses prevent you from putting your eyes close enough and hit the “sweet spot” of the binocular, the picture shows what happens to your field of view So, if you happen to wear eyeglasses, you have to make sure the eye relief is at least 15 mm. Except in the case of very expensive binoculars, field of view and eye relief work against one another. If the manufacturer increases the one, it is at the cost of the other. If you do not wear glasses, you do not need good eye relief. If you do wear them, you have to be content with a compromise – unless you’re willing to dig deep into your pockets.