Interesting finding since I've never really used the tubes with my 150mm, though I'm surprised to hear that they flex that much. Then again the 150mm should strictly speaking, be used with its tripod collar rather than the tripod mount on the camera body (with the lens supported by your hand if hand holding). I know several photographers who even use them on the big 500mm f4 lenses for closer work and provided that the lens and camera are properly supported then you shouldn't get any problems with the kenko tubes.