Does anybody have any experience with this lens? (ideally Nikon mount) I might be able to get my hands on the manual focus version of this beast. I know it's built like a tank (and weighs as much as one too), but haven't been able to find much more on the internet. I did find some info on the autofocus version, but I'm not sure if they are basically the same product or not. Also, what would be a fair price to pay for this lens? (It's in optimal condition, and comes with hard leather case, skylight filter, screw-on hood, and lens caps.) Any help would be much appreciated.