Quote Originally Posted by mfbatzap
so do I need to keep my shutter speed abv 1/100 if I shoot with a 100mm lens? hehe.. sorry for hijacking the thread abit.. wanna learn more too
Using a telephoto lens would introduce other blur (ie hand-shaking), that may or may not be obvious. The problem with telephoto, in my opinion, is that if the subject is a long way away, you hardly have to pan, and therefore the background isn't too blur.

In the following 2 scenarios:

1) CAMERA -------- SUBJECT ---------------------- BACKGROUND

2) CAMERA --------------------------SUBJECT------BACKGROUND

If the subject is moving at the same speed in both scenarios, you'd have to pan much faster in scenario #1 to keep up. Thus the background would likely be very blur. Perhaps you'd also need a faster shutter speed in #1.