Thanks for the replies and link to your test shots, which I examined. I did an informal test today, 600mm lens mounted on a heavy tripod, mirror lock-up, identical exposure but with IS on and IS off. Again, the results were not absolutely conclusive, but in approximately two thirds of the test shots the images taken with the IS turned on appear slightly sharper. I plan to continue to explore this, the lens was so expensive I want to learn how to take full advantage of it's potential. Are there any other Canon forums someone can suggest on which I could also post this query?