According to Canon, if you are using a tele-convertor (TC) with your telephoto lens, which is really a silly statement because you wouldn't be using it with your wide angle lens, it is important to mount the TC to your lens first, then mount the collaboration onto your camera body so that the camera receives the proper communication. If you attach the TC to the camera first, and then the lens after that, the camera may get an erroneous signal that might be deleterious to your auto-focus, or even image quality (IQ). They didn't specify if the camera was on, or off, as if the issue might arise either way. Time for some experiments, if I wasn't so lazy. I need a blood transfusion from Charline because I don't do speed ... not in these shoes.