Seems like as good a thread as any to ask a question about diopters I never see discussed anywhere: I get the need for a diopter if you can't properly see at the effective distance that the viewfinder appears at. But does it change the apparent focus of the scene in the viewfinder, and/or the accuracy of the focusing aids? Let's say for instance that I know my eyes are no longer perfect with age, though they're pretty good. And when I look in my viewfinder, I can perfectly bring into focus the focusing screen, the metering needle, etc. But when something looks in focus on the screen, or the two halves of the split finder match, etc. is it definitely in focus, or is it off by the amount of error in my eyes? I guess my question is, am I looking a projected image on a surface, so it's a question of whether I can properly focus on that surface (I can)? Or is it like at the optometrist's office, where they put those lenses in front of your eyes, except in this case I'm using the camera lens to correct my vision, so the net result is I'm going to make focusing errors because my vision isn't perfect? I think it's the former, but I'm curious to know if anyone knows the answer for sure.
No! An eyepiece mounted corrective lens simply and only allows you to see the focussing screen clearly (assuming it is the proper perscription for your form of parablepsy).