It seems worth quoting verbatim what Irving Penn says about portrait photography.
The following is from a lecture he gave at the Wellesley College photographic symposium in 1975 ...
"In portrait photography there is something more profound that we seek inside a person, while being painfully aware that a limitation of our medium is that the inside is recordable only insofar as it is apparent on the outside ... I have at times seduced myself into a mystical belief in the penetrative power of the camera, but reflection always brings me back to accepting the picture process as simply the bounce back of light from a momentary arrangement of atoms that are a face. But that is not to say that the power of a tender word, or a clumsy one, to affect those atoms can be overstated. When light and the situation of the portrait picture are found and the sculptural arrangement made, it may be that the word is after all at the heart of the whole thing ... Sensitive people faced with the prospect of a camera portrait put on a face they think is one they would like to share the world. ... Very often what lies behind the facade is rare and more wonderful than the subject kinows or dares to believe."
In regard to portrait photography, such words seem profound and match the gravitas of many of his portraits.