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"RE: How to get sharp pics with the D800?"
I think that for F mount lenses, you need to send the lens back to Sigma for a firmware update. I don't believe that any of them are user upgradable in the same way that their 4/3rds lenses are.I'd follow the steps that Stan has outlined first before seeing the camera off to Nikon or the lens off to Sigma.
Don't visit my Nikonians gallery yet. It's empty.