my figure drawing teacher does this sort of portrait sketch for class demos, out of his head all the time... he also has about 35 or 40 years of experience drawing and painting figures.
(I googled a similar image - I'll take a photo of what he does next time he does it)
Sometimes he does things from a model, sometimes from his head, kind of depends on the lecture and focus of the demo.
Part of the class we are learning observation, and part of the class we are learning construction, so that you can draw 'from your head' too.
Even in beginning drawing and beginning figure drawing I'm already noticing what Kev is talking about, how you just don;t get the same information from the photo as you do from the observation. I think for one, our stereo vision plays a big part. Photo seems to give us a flattened view of what we may see out of one eye. When we draw from life, we sort of process what we see with both, and draw our impression of it. Something that my teacher often tells us is 'don't be afraid to exaggerate a bit' (he corrects us if we do too much too). Sometimes you need to be there and see the model from other subtly different points of view in order to exaggerate what seems important, or what we want to accent.

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