About a 1 hour sketch/study - I really wanted to emphasize the orange bounce light and slight subsurface scattering in the nose and lips. I'll do another quick study with a heavy subsurface light source..
Aaaaannd completely unrelated to drawing, I want to document how to use Z-depth for focus and fog - before i forget..
Paste the zdepth as a new layer on top
go to channels window and Ctrl+left click rgb layer
use this as new alpha layer
deselect and select regular layer
click on the image where you want it in focus
adjust radius to increase/decrease the amount of blur needed
cool! now for fog.. not quite sure, but invert zdepth and mess with blending modes such as exclusion