Thanks buddy! Yes I sorta figured that, but I don't want to bothering using extension tubes anymore. I'll just use my 105mm micro for my macro needs. It works very well without any hassle. I just tried the 80-400mm for the heck of it out of curiosity.
The closest focus distance is about 6ft for your lens! But it will focus closer if you put a macro tube on and hand hold to see how close it will focus. Then put on your tripod once you know the range.