I have read many times on the forums of the necessity for micro adjustment of lens to camera body. In reality this seems a very essential exercise to carry out in order to achieve optimum focal fidelity, but one thing puzzles me; presumably the difference is measured in fractions of millimeters and would be obvious with a camera/lens mounted on a tripod, but I hand hold my Canon 100-400 and I would have thought that the body could move back or forward by some millimeters between achieving focus and the image being taken, resulting in a front or back focus effect. Could somebody with more technical knowledge than I have please explain?