Perspective is due to subject distance, not focal length.
"Note that linear perspective changes are caused by distance, not by the lens per se – two shots of the same scene from the same distance will exhibit identical perspective geometry, regardless of lens used. However, since wide-angle lenses have a wider field of view, they are generally used from closer, while telephoto lenses have a narrower field of view and are generally used from farther away." http://en.wikipedia.org/wiki/Perspective_distortion_(photography)
The reason 85-105mm lenses are favoured for portraiture, is because the distance needed for "head & shoulders" framing provides a slightly telephoto compression effect that is considered flattering. Note that using a crop camera requires adjustment of the focal length to provide the same perspective and framing. APS-C classic portrait lenses would be in the range of 55-70mm.
Some people define a wider range for portrait lenses, e.g. 70-135mm FF equivalent. I find my FA 50mm (75mm FF) uncomfortably close for portraits, but not because of perspective. I'm uncomfortable because I feel like I'm crowding the subject.
Last edited by audiobomber; 12-21-2014 at 07:03 AM.