オリジナル投稿者： Andrew ,
Error 99 showed on my EOS350d when the body could not set the desired aperture in the lens. This occurred on my old Sigma70-300 (1999) on anything except wide open and was due to the lens being old and 'not compatible' with the newer body. I was able to take the lens to the Sigma repair agent and have it re-chipped to compatible firmware. The lens worked perfectly since including on my new 550d.