28mm VF with diopter
I don't think it's a diopter correction, how about a distance correction. As you move closer to your subject you rotate the ring that in turn moves the back of the VR up so it points Down a little so that you have a better idea of what's really in the picture. It just takes the place of those extra little lines in the VF that is set for close ups.
|Fangorn Forest by cand1d|
|Yosemite Falls with Moonbow by Jonathan Shapiro|
from Best Landscape of the Week 4