I have the exact same problem. I recently bought a Sigma 150-600 with a Canon to Fuji converter. And guess what? My shots are pin sharp 90% of the time! As for my Fuji, still can't get good shots with it... So it's not you neither it's me. It's the lens. I guess some of us haven't been lucky and got a lens with soft focus problems. Because I've seen other photogs posting shots taken with the exact same lens (Fuji 100-400) and they got tremendous results.
Now that we think it's the lens, any suggestion for testing it?
