So I would like to do some wildlife photography with my NEX5n. I have been buying Nikon AI-s lenses from eBay and getting great results from them. Both of these lenses are manual focus which is going to limit me a bit but that's okay as it allows me to learn. Also it allows me to get better glass for the money spent. With that said, there are reviews out there about the Nikon lens giving it good marks. The Sigma on the other hand I haven't been able to find. I have found reviews on the AF versions but not the older MF's. I wonder if the optics are of the same quality or if they have changed? Which would give me better images? Is it better to have the extra 100mm? Also, shooting with this sensor doesn't that make the 400mm actually closer to 600mm and something similar with the 300mm? If so the 300mm would probably be plenty.... Help....