I just got my hands on an old Nikkor ED-IF 300 f4.5 and I love it on the NEX-5. I believe it's the lightest and smallest 300mm Nikon ever made, weighing in at 990 grams, just over 2lb. It's hand-holdable if you're careful, use a high shutter speed, and practice a bit. By the way, the lens itself is not marked IF, only EF. But you can tell it's the IF version if the lens doesn't extend when focusing and the aperture goes to 32 rather than 22. Don't know how hard these are to find. I was fortunate to get one from an older family member.