OK........ So I have a nikon d40 and I am soon to buy an af-s Tamron 70-210mm f. 2.8, for $670-ish, new. Because it is a f. 2.8, i am looking to teleconvert it into a 140-420mm f. 5.6. I am looking to buy a white Tamron teleconverter for it, HOWEVER, I have heard that the only teleconverter that would work and AF on it would be the kenko dg af-s teleconverter. I really want the Tamron tc, but not at the expense of functionality, so, any thoughts? please?