Here's an article about eye relief I've been meaning to write for a while. Though I suspect most birders understand how binocular eyecups are used for eyeglasses and non-eyeglasses wearers, I thought I would explain the why of it and illustrate different types of vignetting that happens when you're not getting the proper amount of eye relief.
Eye relief is a fixed distance from the eyepiece lens to its exit pupil. To get sufficient eye relief, your pupil needs to be placed at the end of this fixed distance (right at the exit pupil). If binoculars were designed for non-eyeglasses wearers and lacked adjustable eyecups, the fixed distance wouldn't be long enough for people who do wear glasses because their glasses prevent them from getting close enough to the eyepiece lens.
The solution is two-fold: First, give binoculars longer eye relief than what a non-eye glasses wearer needs, and second, mount an adjustable eyecup to the eyepiece. This way the binocular's exit pupil can be reached by your pupil whether or not you're wearing eyeglasses; one merely twists the eyecup up or down to get to the exit pupil.
When your eye is too close to the eyepiece lens, vignetting in the shape of a crescent shadow will appear and shift around the circumference of the field stop as you dart your eyes around. The width of the crescent shaped shadows might also fluctuate as you move your eyes. In the case of too much eye relief, your pupil's edge is the source of the obstruction. This happens when non-eyeglasses wearers forget to twist the eyecups out. Note: The field stop remains sharp with this form of vignetting.
When your eye is too far away from the eyepiece lens, diffuse and uniform vignetting will appear around the entire circumference of the field stop. The further you pull your binocular away from your face, the more profound this effect becomes and the more constricted your field of view. In the case of insufficient eye relief, the source of the obstruction is that light rays at the binocular's exit pupil are missing your pupil. This happens when an eyeglasses wearer uses a binocular that does not have sufficient eye relief (at least 15mm). Note: Though the field stop edge appears soft from this form of vignetting, you're not actually seeing the binocular's real field stop.
When your eye is the correct distance from eyepiece lens, the field stop will appear sharp and you can see the entire field of view the binocular offers without any vignetting, shadowing, or obstruction; all necessary light rays exiting the eyepiece enter through your pupil. This is sufficient eye relief!
Article and photos contributed by Mike McDowell, an avid digiscoper, citizen naturalist, and Eagle Optics employee.