After getting a basic values and structure study out of this one, I pushed it further and tried to mimic how Jackwhat does some of his face studies with very crisp and clean eyes. More or less successful, but it could be pushed a bit more. Using the circle lasso for the iris area seemed to help. I really should use more of the digital tools available to me, like the lasso and layer masks.
I also restumbled upon Hannes' blog today. That dude is a beast.