The 55-250mm IS in current use is not really long enough for many birding shots. With a budget, of, say, $300, and willing to buy used, what is the best choice? Options appear to include the old Sigma 170-500mm, the old Tamron 200-400mm, the old Sigma 135-400mm, what else? Or would the 55-250's IS for handheld shots plus some cropping, mean that it remains the best choice? Or for that matter, is the Tamron at 400mm and cropped effectively sharper than the Sigma at 500mm? By the way, I think decent auto-focus performance is a must. Yes, I know there are some old Sigma 400mm f/5.6's out there, but many of them seem to have issues working with more modern bodies. Thanks!