When the first external recorders for DSLRs came out in 2011, there was a drawback: they didn't work with DSLRs! Now, slowly, that is changing, but the 5D mk III is the big one and now, with its newly released firmware, a clean HDMI output is available to record externally
Imagine the disbelief when one by one, the manufacturers of external recorders designed specifically to work with DSLRs realised that the HDMI outputs were contaminated with all the text and symbols that you get in the camera's viewfinder. And that wasn't the end of it. Sometimes what was output through HDMI was of a lower resolution than the camera's internal recording, or, in the case of the Panasonic GH2, contained a strange, unfamilliar pulldown cadence.
A clean output
But now, belatedly, these HDMI issues that perhaps shouldn't have been there in the first place, are melting away, to reveal a clean output that's ready for recording directly as uncompressed video, Apple ProRes, or Avid DNxHD.
Here's an admittedly rather sales-y video from LensProToGo.com that explains exactly how to connect an external recorder to a 5D mk III, and how to get round the slightly surprising nuance that the 5D mk III doesn't output audio through its HDMI connection.