I'm not terribly short-sighted, but I thought I might put all chances on my side by ordering a diopter eyepiece for my Nikon FM2n. My prescription for the right eye is +0.5 sphere. The astigmatism is rather low (-0.5 cylindre), so I don't take it into consideration. Logically, I told myself, I will use the Nikon +0.5 diopter eyepiece. They say that +0.5 is the combined lens/finder dioptry, so I reason that it means that the entire camera optical system between my subject and my eye is equivalent to +0.5. Lucky me, I need +0.5 in my right eye to see properly. That's what I have in my glasses. Now here's the catch: I bought the eyepiece, put it on my FM2n, and in fact it makes viewing worse. I checked very carefully again and again, putting the camera on a tripod and not touching the focus, and still, it looks worse with the eyepiece correction. Did I make the wrong calculations based on my prescription? Or is it just the case that my brain is not used to the corrected view, so he has a hard time processing the new visual data? I fail to see how the fact that the viewfinder is -1.00 by default has anything to do with my problem.