Because of their focus on the head and (particularly) the face, source material needs to be very carefully selected if the portrait is to be truthful and say something about character, circumstances, personal history, etc.

 

To achieve this, I believe that it is better to avoid situations where the subject might feel closely scrutinised and become self-conscious. This is why I prefer to use casually or covertly captured photographs or video, where the person might be distracted, off-guard and more naturally themselves. Ideally, they would not be aware of being photographed or even observed.

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