I currently have a Nikkor 80-200mm f/2.8 AF-D and sometimes I need more range. I was looking for a teleconverter 2x that will STILL LET ME US AF. I was looking at this.... http://www.bhphotovideo.com/c/product/371030-REG/Kenko_K20PRO300DGN_2x_Teleplus_Pro_300.html Which would make my f/2.8 a f/5.6 right? I heard that AF stops working after f/5.6? Why is this? Anybody have experience with this TC?