I recently bought this lens. Canon Professional Services recommends shutting the Image Stabilization OFF when tripod mounted. The lens instruction manual is vague, stating that IS sometimes helps when tripod mounted. I read on another online forum that the IS senses when the lens is tripod mounted and compensates for such. My own tests were inconclusive in that some tripod mounted images with the IS turned on were sharper, while other images with the IS turned off were sharper. Does anyone have and feedback on this? Thanks. Mark W.