オリジナル投稿者： whittsmithphoto ,
With a 30 D and a 20 D, it turned out to be the lens in my case. It was not able to communicate with the camera. For this, you will need two lens's and a camera body or two camera bodies and one lens to do a process of elimination. Take the camera and lens that is getting an error 99. Put the other lens, in place of the lens in question, on the camera body. If it works then it should be in the lens that you are having problems with. Try the same thing using an additional camera body if you don't have a second lens. This should help you to narrow the issue to either the lens or to the camera.