Firmware 1.2.1 provides an uncompressed 4:2:2 video output while allowing for simultaneous viewing on the built-in display. Like the EOS-1D C, you can also embed timecode via the HDMI output to a supported device, like an external recorder. This gives the 5D Mark III many more recording options, as you can now use your favorite external recorder to achieve higher data rate recordings in a codec streamlined for editing.
In addition to the clean HDMI output, the Mark III also gains some autofocus updates useful for those using Canon teleconverters. You can read the full release notes and download the free update directly from Canon's site. Have you already updated your camera with the new firmware? Let us know what you think in the comments