Same person’s portrait taken at different focal lengths for the sake of comparison. By Stephen Haywood, who has more thoughts on focal length. Click to enlarge.
A couple things pop out:
Wide angle focal lengths
19mm and 24mm – The ultra wide angles look terrible. Wide angles distort perspective by exaggerating distance relationships. Because of wide angle distortion her nose sticks way out in front and her hair look like it’s receding from her forehead.
35mm – Not quite as bad, but still terrible.
Normal focal length
50mm – This is the “normal” focal length – it roughly matches the human eye in terms of field of view and depth perspective. If you want to know what she actually looks like in real life, this is it.
Telephoto focal lengths
70mm and up – Continuing slight improvements on reality. Whereas wide angles exaggerate relative distances telephotos compress them. Her nose gets smaller in the Z-axis. More surprisingly, her long hair gets pulled forward around her face and her hair even seems to get fuller. My wife liked the 200mm best.
Small wonder people like the 85 and up range for portraits. Most of the benefits seem to be in the 70 to 100mm range. The longer telephotos give a bit more of the effect and allow more working distance between photographer and subject. Haywood notes that to get a full frame head shot with a 50mm he had to get within about a foot of the subject.
P.S. Those focal lengths are 35mm film equivalent focal lengths. If you’re shooting a crop sensor DSLR multiply as needed. I shoot a 35mm prime lens all the time, but on a crop sensor Nikon it’s effectively a (35mm x 1.5 crop factor) 52mm.
I have a big honking 70-200mm zoom for portraits, but this is making me want a longer prime like the 50mm or 85mm (which would be 75mm and 127mm on my camera). The advantage over the 70-200mm is for when I didn’t want to carry a 3 pound monster lens or need more light or more depth of field, or when I’m in a smaller space – the minimum focus distance for the 70-200mm is 5 feet.