For every pair of binoculars, there is an optimal distance between your eye and the eyepiece, which is called eye relief. The eye relief of a binocular is the distance your eyes are allowed to be away from the eyepieces allowing you to still see the full field of view without any part being blacked out. The eye relief is crucial to people wearing eyeglasses, since the glasses prevent your eyes from getting closer to the eyepieces of the binocular. If you are nearsighted or farsighted your problem can be corrected by the focusing action of the binoculars, but not if you happen to have other eye problems, like astigmatism. With this condition you will need to wear your prescribed glasses, whether you like it or not and if the eye relief of the instrument is not sufficient, the field of view will be severely impaired, a phenomenon known as “vignetting“. The image you’ll see will also lack sharpness.
So, if you have to wear your glasses using a binocular, make sure the eye relief is at least 15 mm.
All in all, magnification is yet again the determining factor. The Nikon Monarch ATB 8×42, 10×42 and 12×42 are exactly the same, except for the magnification. The differences in exit pupil, field of view and eye relief are all a consequence of the difference in magnification.
In this case, the higher the magnification, the shorter the eye relief:
The Nikon Monarch (7294) 8×42 has an eye relief of 19.6 mm.
The Nikon Monarch (7295) 10×42 has an eye relief of 18.4 mm.
The Nikon Monarch (7296) 12×42 has an eye relief of 15.4mm.
The budget priced (less than $100) Nikon Action 7×50 Binocular has an eye relief of 20 mm. That goes for the more expensive ($300+) Steiner 7×50 Marine Binocular as well.
So, if you’re wearing eyeglasses and have to use them when using a pair of binoculars, rather stay with the ones with lower magnification. For a review of binoculars suited for wearers of eyeglasses, follow THIS LINK.