[lars_bergquist]

I really don't think some focal lengths are better for portraits than others are. Do you want those passport-style 'big heads', 1930's style? Or is your ideal Holbein's full-length portrait of king Harry, aggressive glare and all? The full length likeness was once the most prestigious style; only princes could afford it. You can in fact go even further, for there is a genre that may be termed 'environmental portraits', the person in telling surroundings -- as long as the person is the real subject.

So it all depends. You can go all the way from say 28 mm to 135, or even farther! The thing to remember is never to approach an adult person closer than about 2 meters or 7 feet. Not because he or she will bite, but because a close approach (NOT the same as a short focal length!) will lead to perspectival distortion. Infants and young children have flatter faces and can be photographed at slightly closer distances without exhibiting the dreaded Zeppelin Nose Syndrome.

The old man from the Age of the Studio Portrait