I bought this "Sigma 70-300mm f/4-5.6 DG for Nikon" on ebay to use with my Nikon D40. The Sigma website states "The built-in motor allows autofocus with all Nikon digital cameras." Well, it doesn't. Do any of you know if Sigma made a "for Nikon" lens like this that did not autofocus? Thanks in advance for all responses.