Originally posted by mtroute really, so when I throw on a 100mm lens, I am not getting 100% closer (magnifying) to the image than I would be with my 50mm? (in film terms for simplicity)
Obviously. You're only getting closer if you physically move closer. A longer focal length lens will produce an image with a narrower field of view than a shorter focal length lens, and that's what makes the subject look bigger on the print. Projecting the image onto a cropped sensor and producing a print of the same size from that sensor has exactly the same effect.
Now, it's entirely possible the longer lens will do so with more resolution than the cropped sensor, but that depends entirely on the specific lenses invovled as well as the specific film you are shooting and the resolution of the specific sensor you are using. Without knowing those variables, then indeed, for all practical purposes, cropping *is* exactly the same as using a longer focal length.
Back to the original question: a 50mm lens is a 50mm lens. So no surprise that the 50-200 at 50mm produces the same image as the 50/1.7. Doesn't matter if the lens was created for film or for digital, 50mm is 50mm.
the relevant comparison is not different lenses on the same camera, but one lens on two different cameras. Put that 50mm lens on the film camera then on the digital, and see which appears to have the longer focal length. You'll find that for a shot where the 50 on film would have shown head and shoulders, the same 50 on digital will show face only, etc.