Canon rebel xti Tamron 18-200 lenses, error 99 every few clicks

This failure is mainly due to two basic things:
1 .- the camera and lens not making good contact;
2 .- the lens is not canon, and sometimes gives that error of incompatibility.

In short, this failure appears when "not understand itself" the lens and camera. Firmware may be due to the lens that dont fits the camera, but obviously not your case, because before you no wrong. So all you have to do is carefully clean all the contacts, both, the body and lens and problem solved.

If this not work, necesarily you will need to contac yourt Canon local service. In addtion, I suggest read this article: 50mm-lens-contact-points

Hope this helps.

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http://photography-on-the.net/forum/archive/index.php/t-228087.html