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I was wondering: is the difference between a 50mm f/1.8 lens and a 85mm f/1.8 lens just your distance from the subject? Let's assume that distortion is minimal; both lenses are of high quality. Does that mean I could take a step or two back or forward and use only one lens?
Note that I'm looking for scientific answers.
Perspective is a function of distance, not the angle of view of the lens. You could crop an image taken with a 50mm lens and get the same result as using an 85mm lens, but walking closer with the 50mm would give your shot a different perspective.
The whole scientific answer I'm not qualified to give, but there is a Wikipedia page on this subject: http://en.wikipedia.org/wiki/Perspective_distortion_(photography)
Short telephoto lenses (in the 80-100mm range) are often used for portraiture due to their ability to compress perspective. This flatters the subject by making features (the nose in particular) stick out less. But to back up what David has said, perspective is a function of distance. You can get the same perspective with both lenses if you stand in the same place.
However if you stand in the same place with the 50mm lens, using the same f-stop, and crop you will achieve greater depth of field than the 85mm (on account of it's shorter focal length). This is often seen as a disadvantage for portraiture.
If you get closer with a 50mm lens instead of cropping then you will have almost exactly the same depth of field, but the 85mm lens will have a greater degree of background blur than the 50mm as f/1.8 @ 85mm means a 47.2mm aperture, whereas f/1.8 @ 50mm means a 27.7mm aperture.
In my opinion, for people photos the only reason to prefer the 50mm f/1.8 lens (apart from cost) is if you foresee situations where you can't back up to get the subject in (i.e. group photos indoors) otherwise the 85mm + backing up would be my choice.
Otherwise a 50mm is a very versatile all rounder (and came bundled with many SLRs before cheap zooms were available).
