Im looking to get an extender for my 70-200 F/4L lens. Is there any difference other than the extra weathering seals? I am going to be just using this is warm weather shooting my sons golf. I have shot with the lens and it is a gem. Thanks to all for your help a few months back in helping me pick one. With the Rebel XTI, 70-200 and a monopod this has produced some awesome images. Just looking to get a little bit more out of it.