Originally posted by Parallax Wheatfield, you are right. A 35mm lens is a 35mm lens. It has no idea what size sensor (film or electronic) is behind it. 70-80mm is a better portrait lens than a 50mm on a 35mm camera. using a 50mm on an APS-C sensor does not make it take on the characteristics that a 75mm has on full size. If you want those characteristics you would still need a 75mm lens.
There was a thread a while back where someone said that to minimize shake effect with the old maxim "shutter speed should exceed focal length", you should figure in the "crop Factor". i.e. if you have a 200mm lens on FF, use 1/200th+ shutter speed, but on APSC you would need 300+. I don't believe that as the lens produces an image circle of a given size focused a a specific point; it has no idea (and doesn't care) how much of that image is being recorded.
While there is no "crop factor" as you say, and it is only a relitive comparison of formats, there is one point that MUST be considered. This is with respect to the "golden rule" of shutter speed = 1/focal length.
This "rule" is applied directly to 35mm format, and the ability to eliminate camera shake with respect to acceptable image sharpness for a full frame (35mm format again) 8 x 10 print.
If you enlarge beyond that, you must use higher shutter speeds. Therefore, if you make the same 8 x 10 inch print with a smaller sensor, you are using a higher enlarging magnification and you should consider this increased magnification, whether you call it crop factor or something else, in your estimation of shutter speed to produce acceptable image sharpness.
if not crop factor, what would you call it?