Czech photographer Dan Vojtech conducted a fascinating experiment to show how the focal length can change the shape of a person’s face. Using lenses that increased incrementally from 24mm to 200mm, Vojtech demonstrated, via a clever GIF, that the camera truly can add ten pounds. That being said, it can also subtract those same ten pounds much in the same way.
Its always difficult to answer what is the best focal length for a portrait. Here is small preview how different focal lengths can change look of a face.