Last week I finally got a Canon 500 f4 IS, so far I have been very impressed with the lens, speed of focus and sharpness are excellent. I can already see how useful IS is when shooting from the car, however I do have a question or two. I have been told that it is good to leave IS on even when using a tripod, is this right? Also the booklet says that I should switch the lens to IS 2 when taking panning shots, when do others use this? Just for flight shots or also when panning with a running or swimming bird? Any (other) advice on using this lens would be great, I really want to get the most out of it.