Originally posted by TomInJax I disagree. It isn't because of the focal length that perspective changes, it is distance to the subject.
For example, if you are taking a picture of someone from 15 feet away with a FF camera using a 46mm lens. Then you picked up a K10D with a 31mm lens and framed it exactly the same as you did with the FF camera with the 46mm lens, you would still be 15 feet away. All things otherwise being equal, the portraits would be identical.
.
You may have something there...any physicists in the crowd? I know that what you are saying is true for flat subjects taken straight on and for all subjects as distance approaches infinity.
I recently did a series similar to what I described because I doubted the very assertion that I made in my original response. Strangely, the shorter focal length images still exhibited the typical "wide-angle look" (exaggerated perspective and greater DOF). The normal still looked "normal" (though cropped) and the longer focal lengths still looked like they were taken with a telephoto (flattened perspective and narrowed DOF).
Before I got my K10D, I figured I would mount up my 28mm and use it as a short normal. I tried this for awhile, but was not happy with the results. The 28 is rectilinear and has very low distortion, but regardless of FOV, the pictures never really "looked" right. Strangely, they looked like crops from a picture taken with a 28mm lens!
In any case, I would suggest that the poster try it out. If the results are good...go for it!
Steve
BTW...I would have posted my test pictures, but I deleted them...perhaps I will redo the test?