Hi everyone, and merry christmas to all my friends on here. Just a simple question about my extension tubes I just got. I ditched my other cheapo ones and went for the kenko extension tubes. The cheap ones I bought last year, I kind of was nervous putting them on and off my macro lens because they just were soooo terribly tight that I was afraid I was going to damage either my camera or my lens. Anyway, just got the kenko tubes and while they are much easier getting on and off, I am finding that I can't manual focus. It won't let me turn my focus ring on my 105 macro. It feels like I need to force the lens to focus,,and I don't want to do that. It sounds almost like a grinding sound. I made sure I had my lens set to manual focus, so that isn't the problem...Not sure what else to do. It will let me manual focus if I have only 1 of the tubes on, but not with two...I didn't even try it with all three...Any ideas?