Matt Laur , May 19, 2009; 01:28 p.m.
Don't confuse field of view with perspective. They're not the same thing.
The ONLY thing that changes perspective is distance between you and your subject. Forget the lens for a moment.
Imagine that you're looking at someone's nose from two inches away. It will look large, compared to their ears. Of course, your brain naturally compensates for this to a certain extent, and you don't really think about it much when you're in the flesh looking at someone. But if you took a photograph of their nose from a few inches away, you'd see and really note that the nose looks large, compared to the ears. If you use the same lens and back up a few feet, the nose and the ears are (compared to you) much closer together, and the perspective effect is dimished greatly.
Here's why: when you eye is two inches from a nose, the ears might be six inches from your eye. That causes perspective "distortion" (it's not really distortion, per se) just like when you're looking at one automobile that's ten feet away, and another that's fifty feet away from you. If you took the picture of those two cars from 200 feet away, the difference in distance between the two cars would be much less meaningful, and they'd seem more in porportion to each other (just like you'd hope that noses and ears would be)... but if you shoot that closer car from five feet away, it will look huge compared to the one that's now ten times farther away. Faces and body parts work the same way.
Focal length has NO impact on perspective. It only determines how easy it is to fill the frame with your subject at different distances. From 15 feet away, your 35 and 50 lenses will produce the same looking person. But the 50 will fill the frame better with their body.
But if you try to fill a frame with a person's shoulders and head using a 30mm lens, you're well within the distance where perspective will start to make the closer parts of their body look larger compared to the rest of them. Hope this helps.