Determine the optimal dioptric correction

Question

I just got a DSLR (a Nikon D610). On it, a dioptric correction is available, apparently ranging from -3 to +1 (cf. page 297 in the D610 manual). I'm wearing glasses, so I'm thinking: "Yay, I'll be able to shoot with my bare eye and see the whole viewfinder with ease!"; I've been shooting film for years, but with my glasses on.
However, it seems that I can't get the focus as sharp as with my glasses on, even on the -3 correction. Which seems to me very odd, because here is my glasses prescription: I'm "aiming" with my right eye. It's just that maybe I can't read my prescription right! If I understand, I need a -0,5 correction, so within the range of the D610.
Is there something that I'm missing? I also have an astigmatism, so perhaps that's why I can't get the focus right with only a "simple" correction?
I saw that an accessory exists to correct that: the Nikon DK-20C. It exists with a few corrections, but I want to know if it'll be useful before I buy one.

Thanks!

Answer 1

Your right eye has astigmatism – 0.5 -1.00 axis 160°. The common lens is a section of sphere, think of a slice off of a crystal ball. Your correction is more like a bore hole in a cylinder. In other words, two different curves, one on one side of the cylinder, another curve on the other side. The diopter eyepiece correction is for the simpler slice off a sphere. If you have a conversation with your eye care professional, you can obtain a tiny lens that will overlay on your camera’s eyepiece.