I have decided that I cannot afford a Nikon 500mm f4. Even used. So, I have set my sights on the Sigma 500mm f4.5. Every review that I have read is positive and so I feel pretty good about the lens. BUT, there are two Sigma 500's. One is a new "DG" type, and one is not. The price difference is $700 ($2999 vs. $3699). From what I have read, the "DG" glass coating is mostly hype, and not worth the difference. My question is: Is the "DG" lens a replacement for the older model? SIGMA4LESS.COM says they have the older one on order, but when I pressed the salesperson, she did not know if the DG is a replacement and whether they will get a shipment of the older models. Any insight would be appreciated.