I had been taking lightly the recommendation that Image Stabilization (IS) be turned off on my Canon 100-400mm f/4.5-5.6L IS lens when using on a tripod. After having many soft pictures, I did another side-by-side test on a real subject.
The images below were taken of a snowy egret at about 50ft with camera mounted on a tripod. In both cases, I used back-button focus in AI Servo mode. Before each shot, I unfocused the lens, let AF acquire focus on the feather detail, and let the IS settle for a second before taking the shot. The effect of using IS when "not needed" is pretty dramatic. What I am still uncertain of is what constitutes "not needed". I have taken slow tripod shots where the IS slightly improved the shot. Is it "tripod and fast shutter" or "tripod or fast shutter" that requires turning off IS.