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Your shading approach is good. The transitions from highlight to halftone to core shadow are good. Knowledge of the face and its planes are important for two reasons: Occlusion/cast shadows and expressions. Drawing a convincing nose, for example, is next to impossible with the finest shading without knowledge of the planes and how they interact with light. I think your drawings look a bit uncanny because normally an entirely black background has no bounce light (think of the moon in the sky) so try to have them on normal backgrounds for these critiques. I'll leave you with this excellent diagram by Andrew Loomis from his book about heads and hands. It's a timeless resource and I highly reccomend it. You don't have to buy any book to know the planes of the face though, just look up a 3d asaro head on google and play around with the models, drawing them in unique and interesting angles, understanding the relationships in size and how the different planes fit together like pieces of a puzzle.
