I have a Sigma 500mm 4.5 EX DG lens that I have always used with a Sigma 1.4 EX DG teleconverter that used to auto focus perfect. I sent it to Sigma in New York after years of service. When I received it back it would not auto focus with the converter. I called Sigma and was told it was not sopposed to auto focus with a teleconverter. I have never heard of this every where I look and hear people are using converters with 500mm 4.5 with auto focus. HELP