When introduced to the realm of optical jargon, eye relief is a number shrouded in mystery for newcomers. So, as an optics specialist, what might I tell you about eye relief? For one, every binocular and spotting scope eyepiece has it, but generally only eyeglasses wearers need concern themselves with it. On average, an eyeglasses wearer needs at least 15mm of eye relief. So, it can be measured, and yet it isn't visible. It will become obvious if you don't have enough of it; your field of view will be constricted or vignetted. You can also have too much eye relief, causing you to see crescent shaped shadows that seem to move around in your optical field. So, what exactly is eye relief? One technical definition is the distance (expressed in millimeters) from the last surface of the lens of an eyepiece to the plane projected behind it where all the light rays of the exit pupil come into focus. That's a lot of optical terminology in just one sentence for non-techies.
Binoculars aren't made differentially for or distinguished by eyeglasses wearers and non-eyeglasses wearers; a given pair will accommodate both users through the use of a retractable eye cup on the eyepiece assembly of a binocular or spotting scope. When the eye cup is twisted down, a larger amount of the eye relief is available for eyeglasses wearers. When the eye cup is twisted up, less of the eye relief is available, which is fine for people who don't wear glasses. The reason glasses wearers need a little more of the total existing eye relief is because their eyeglasses are in the way so the focal plane can't reach the person's eye. This causes the field of view to be vignetted.
Remember, as a rule, if you wear glasses you'll need at least 15mm of eye relief. However, if your glasses rest further out on your nose, or if you have a strong prescription with thicker lenses, you may need 16mm to 18mm. The good news is that most binoculars today have ample eye relief. Just be mindful that you don't choose a binocular (or spotting scope eyepiece) that has 12mm or 10mm or else you won't be able to wear your eye glasses when viewing through it.