In my post yesterday, some people were having trouble seeing what I was getting at with the full length figures, so here is a better example, concentrating on just a head shot.
http://www.filmphotoacademy.com/blog...e105_thumb.jpg
This is shot at 105mm – everything is well proportioned – she looks human
http://www.filmphotoacademy.com/blog...e070_thumb.jpg
This is shot at 50mm and the proportions are drifting off – by comparing the two images you can see that things are not right at this focal length.
http://www.filmphotoacademy.com/blog...e024_thumb.jpg
And to take it to the extreme – this is 24mm – obviously the distortions are greatly exaggerated – but the 50mm is still showing the same distortion just not as strongly.
The reason for this is not a distortion in the lens, but a perspective distortion. It's caused because nearer objects will always appear larger in the frame. So, when you use a shorter focal length, in order to achieve the same crop you have to move the lens closer to the subject.