I bought a D50 about a year ago. Love it, takes great pics, but there was something that a friend of mine explained and got the same info from the person at the camera store. I don't recall it exactly. It has something to do with the form factor between the digital sensor and the 35mm film. It changes the focal lengths slightly. For example I think an older lens with say 100-200 zoom, would be like 120-240 zoom (made up numbers) on a digital camera.
Here is a link to an article I found that has some good info.
Lens Info