Is the tripod ring removable? I've unscrewed it and it just stops once loosened - can not seem to turn it anymore without "breaking". The "zoom ring" - is it supposed to be smooth? Mine feels a bit tight. I also have a 17-40 f/4L. The zoom ring feels much smoother, but then glass probably smaller so not sure if tightness is to prevent slippage or something. Lens is new so wanted to take care of any possible problems. Thanks in advance.