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From Canon USA
It has been confirmed that auto-focusing may not function at f/8 aperture on some Canon EOS-1D X Digital SLR Cameras with firmware Ver. 1.1.1. We offer our most sincere apologies to customers using this product who have been inconvenienced by this issue. The support measures for this phenomenon are described below.
Firmware Ver. 1.1.1 enabled the center AF point to autofocus when the camera is used with lens/extender combinations whose combined maximum aperture is f/8. On some of the cameras with firmware Ver. 1.1.1, auto-focusing does not function at f/8 aperture. *This phenomenon does not occur with a camera which firmware has been updated by a user, or by Canon service center.
Canon EOS-1D X Digital SLR Cameras with the sixth digit from the left in the body number (serial number) is 4 or 5 are affected.
Example: Body No. xxxxx4xxxxxx or xxxxx5xxxxxx
*Even if the sixth digit from the left in the body number (serial number) is 4 or 5, the following cameras are not affected units.
*Look for a white dot in the battery compartment
Overwriting the firmware activates auto-focusing at f/8 aperture. To accomplish this, please download the firmware Ver. 1.1.1 available for download from Canon Web site, and overwrite the firmware of the camera.