True about the camera-to-sitter distance being the biggest factor in exagerating perspective (making noses bigger, hands bigger, etc.) but this seems to be psychologically mitigated by printing 10x8 or larger, for some reason. A 50mm close-crop portrait will look more unnatural at 5x7 than it will at 11x14. Don't ask me why, though...
The differences between normal and short tele portraits is more obvious if you look at a lot of portraits.
Back when I was advising the high school yearbook we had a shooter in town who used conventional short tele lenses and a beginning shooter starting a studio in in his garage who was using a normal lens (the only one he owned for his 645).
Our policy was to allow the seniors to submit their own portraits for the book.
Looking at any individual picture they all looked fine but laid out 100 at a time on a table and cropped to roughly the same head size, the portraits shot on the normal lens just looked weird. In some cases it was hard to recognize individuals you saw every day.
In that exaggeration of nose size by the normal lens used too close to the subject - in particular - became strikingly obvious.
I think a 50mm can take very flattering portraits if you remember two points: take the picture in profile or with the cheekbone closest to the camera; don't get too close. But even close up and head-on the results can be good:
That's interesting. I wonder if it has to do with the average viewing difference of a 5x7 vs an 11x14 print? Perhaps at a distance our brain is taking in other details and kind of "ignores" the disproportionate relationship we would instantly pick up when viewing a 5x7 print just inches from our face?
Just a guess.
I love the portraits from a Minolta 85mm. Lovely bokeh and tones. Sometimes I think the 100mm or 135mm lenses shot wide open look better for certain tight head/shoulder crops - but it's all in what you want to fill a frame with.
I personally feel 50mm is a little wide for a classic portrait, but it makes a good whole body portrait lens. For whatever reason, I have a hard time deciding between 35mm and 50mm for environmental portraiture, because it seems with a 50mm I can't get enough of the background in to really tell where they are good! I'm always wanting just a little more of the environment in there. So I have to back way up w/the 50! :-)
As already said, it's about flattering perspective, framing and isolation.
I don't think the classic portrait lengths like 85mm and 105mm should be looked at rules so much as classic recipes. They're on the books because they work in a reliable way, but are by no means the only way to whip up something tasty.
Some great portraits are made with 28mm & 35mm lenses. It's really about the style you wish to shoot, my preference is actually a standard 50/55mm lens but I've used everything between 28mm & 200mm over the years