To start this exercise I took two similar self-portraits at the maximum and minimum focal length of a 17-85mm zoom lens. I tried to ensure that my head was about the same size in each of the portraits and the pose was a similar as possible. I really failed on the second of these with the camera as close as it had to be for the 17mm portrait is was difficult control the pose because a small movement of my head made a huge difference to the final picture.
In choosing these extremes I wanted to show the effect of different focal lengths as clearly as possible and it is clear that the exaggerated perspective in the portrait shot at 17mm really produces an unflattering image. The picture taken at the 85mm end is much more true to life but for a Canon EOS 7d with an APS-c size sensor this focal length maybe a little long the portrait to my eyes at least is starting to show flattening of perspective
Finally, I took a similar portrait with the lens set at about 50mm, to me this is a much better portrait there appears a greater sense of depth and I believe that the distance between the subject and sitter was about right for this type of headshot. At 17mm the camera was so close to the subject that it would be distracting to many subjects while at 85mm I felt the camera was too far away for the photographer to really connect with the subject.
For 35mm film and full frame digital cameras 85mm is often regard as the classic focal length for portraits in this case however I really prefer the portrait taken at 50mm. Perhaps this is because of the pose and the slightly different lighting, it was taken a few hours after the first wo when the light in the room was much brighter. People talk about focal length equivalents when using cameras with APS-c sized sensors and taking this into account the 50mm setting would be equivalent to about 70mm on a full frame camera. Although this focal length equivalence is related to the field of view of the lens and not how perspective is distorted as image is projected onto the sensor so standing further away from the model when using a camera with an APS-c sized senor should produce a very similar picture to one standing closer using a full frame camera.