I was looking at a tamron 2X teleconverter or even one of the Nikon ones. I would like one with auto focus as I am using it for taking pictures of birds. I will be putting it on a sigma 70-300mm macro lens. Should I expect a lot of chromatic aberration with this set up? If so what would you suggest for getting out to the 500mm range with out breaking the bank? Thanks for the help.