Eye Relief

Gladiator 7-21x40 Porro Binoculars

The point where the image coming out of the eyepiece comes to sharp focus is physically some distance behind the eyepiece lens. This distance effects how easy the optic is to use for eye glass wearers and how much eye fatigue the optics may cause. Full descriptive text along with how this will effect your view can be found in the Basics II article in the All About Optics section.