After thorough investigation, we have ascertained that some APO 50-500mm F4.5-6.3 DG OS HSM, APO 120-400mm F4.5-5.6 DG OS HSM and APO 150-500mm F5-6.3 DG OS HSM lenses, produced recently, may have a potential autofocus defect.
More on http://www.sigma-photo.co.jp/english...nfo_100616.htm
I'd look into that if you recently bought one of these.

