I'm a bit confused about the eye relief and exit pupil size of rifle scopes. The exit pupil size is simpley the diameter of the front end divided by the magnification. So a 50mm on 5 times magnification will have an exit pupil of 10mms. This is something of a problem as the maximum diameter of the pupil in the eye is about 8 mmms in the dark and maybe 2 or 3 or so in daylight so you can't see the whole field of view the outer parts will be black. On this basis the minimum usefull magnification is at least 20 for a 50 mm scope on a dull day. You can get round this problem by moving your eye back away from the scope and I assume that when you do this the eye is no longer focused on infinity. I have even had this happen on a 20 mms scope at 2.5x on a bright day. Are there any rules about the variation in eye relief needed to make full use of a given scope? Do some of them need less head movement? Any pointers on scope choice? They will all have this problem to a certain extent as we are talking physics.