I've read several times that a 50mm lens will make features like noses noticeably stand out compared to an 85mm lens. However, I'm never sure if the post/article is describing results on a full frame or crop sensor. I know that the longer the focal length, the more the compression to features....because the crop sensor gives a 50mm the effect of a longer focal length, how noticeable is the facial distortion on say, a Canon 550D? I'm considering a prime for portrait work and I'm 'at the crossroads' like all of you who came before me.
I will generally be using the lens for wedding portraits on the beach (and they generally want to get married around sunset, so formal portraits come as I'm losing light — hence wanting a tack sharp f1.8 or below), so it would be handy to have a lens that can take more than just a head/shoulder shot.
Answer
I think Eric's answer is what I have been trying to justify to myself. I'm going to try to explain it, and I hope I get it right (I know people will correct me if I'm wrong!).
That is, it is the distance that determines the compression/disortion, not the focal length. However to get the same framing on a crop-sensor vs. a full-frame sensor, you'd need to change the distance if using the same lens on each camera.
In other words, a 57mm focal length on a 1.5x crop sensor, should provide the same perspective and framing as an 85mm focal length on a full-frame sensor if shot from the same distance.
Now, it seems that 85mm-105mm is most recommended for portraits (full-frame), so a 50mm on a crop-sensor might be too wide (you'd have to get in tighter and introduce more distortion). I think this is why you'll see people using a 70mm (like a 70-200mm) for portaits on a crop-sensor (yielding an effective 105mm on full-frame).
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