My copy of the 100-300 has one very frustrating quirk/defect. It's otherwise a useful lens and does everything others of its sort seem to do (mild softening of focus at the long end at middle and far distances, zoom creep, all the normal stuff) but it needs to be kept after (especially at long focal lengths) to maintain the extension barrel in registration. There's about 1mm of play along the zoom axis at all zoom values including the ends of motion. The extension barrel is heavy enough that any major change in orientation can shift it to the other end of the available play. The result is instant defocusing. Example: focus with the lens level, allow the arm to drop between shots, raise the camera again for the same shot and focus is lost. Does anyone here have any experience of this behavior or of having had this corrected?