Yet again, I'm doubling up with this week's set of life drawings from random photoref. (Someday I'll have to downgrade to a single piece per post, but my buffer of these pieces remains reasonably large.) First above is a set from September 2018, while the second set is an example from January 2017, fairly early on in the effort.
Re: the first one, like that first face especially...

...and the free "Handy" app on iOS has a nice selection of CG heads with variable shading for reference. (I'm rather less thrilled with the CG hands the app features, though.)

Below, enjoy the higher-res, raw and untweaked grayscale scans as PSD attachments.