Any help would be appreciated. Can't afford the Nikon 500mm f/4 af-s and have seen several favorable review of the Sigma 500mm f/4.5 lens. The dg version of the Sigma at $4995 is also too expensive but have found the non-dg version at a couple of places, including Taaffe photo for $2699. Does anyone have any experience with them and is that the best (cheapest) place to buy? Thanks for your help.