The Canon EOS 1D X and EOS 1D C are supposed to be Canon’s hardiest full-frame camera bodies, but it seems the cameras are having some difficulties with autofocusing in cold weather situations. An anonymous source explained to Canon Rumors that the camera’s AF system do not work in temperatures below 0°C (32°F). The AF system purportedly “does not autofocus”, “does not search in AF” or “does not focus in AF search” when exposed to extremely low temperatures.
The source explained the issue is caused by a mechanical defect that causes the sub mirror system to become misaligned. This in turn causes the light rays reflected by the sub mirror to miss the AF sensor, rendering it useless. The full technical explanation follows.
This phenomenon is due to the Locking Claw of the Sub Mirror (mirror for AF) going over the Locking Pin. The Sub Mirror’s angle becomes deviated and the light rays for AF does not fall on the AF sensors, causing the ‘does not autofocus’ phenomenon.
While most photographers won’t have to contend with subfreezing temperature, such as -20°C (-4°F), it’s a major issue for anyone shooting landscapes or capturing polar bears in the arctic. Canon has since corrected the issue for cameras manufactured after January 24, 2013. However, if you’re hanging onto an older camera, replacing the mirror box with a newer version labeled CY3-1661-010 or CY3-1687-010 can repair the malfunction.
Via Canon Rumors