This is a quick demo of using Magic Lantern (alpha version) to get a clean HDMI out from the Canon EOS 5D Mark III. The camera outputs 1080i via HDMI and Magic Lantern removes the onscreen clutter (focus square when not recording and red dot when recording). Still, the actual image has pillar bars, as the full image that corresponds to what the Canon firmware records only takes up a 1620x910 (really, 1620x911) area. To have a 16x9 image, you must zoom in on the HDMI footage, losing resolution.
The other challenge is deinterlacing the 1080i footage if you shoot 24p. I found it to be no problem when shooting 30p (and don't even bother with 720 60p!). But, with 24p, the cadence must be correct. I'm using the BlackMagic Design HyperDeck Shuttle (which I really like, BTW). The BCC (v7) Deinterlace plug-in I am using would not pick up the cadence automatically in Vegas Pro 11. So, it would only be by chance that I would get the deinterlacing correct on the first try. If not, it was a laborious process to trim one frame out of the 1080i footage to try to get the cadence to align correctly. And, I'm still not sure that I've got it correct. There are not any interlaced lines in the footage, at least. Any tips on how to do this more effectively are appreciated.
Finally, my HyperDeck shuttle records DNxHD at 220 Mb/s at 10-bit 4:2:2. I'm nearly certain that the HDMI output is 8-bit. I'm not sure whether the HDMI out of the 5D Mark III currently outputs 4:2:2 rather than 4:2:0. I'm not sure how I would tell. Having said that, there is a distinct difference in sharpening the DNxHD footage in that red doesn't "ring" the way the 4:2:0 from the Canon firmware does. This is what leads me to guess that the HDMI is 4:2:2, but I could be wrong - and had assumed that it was only 4:2:0. The zoom in the last few seconds of the clip does seem to show a bit better resolved color in the DNxHD and/or less compression artifacts (if you can get around the deinterlacing artifacts). However, with the scaling, it is very hard to tell.
The footage here was "graded" in that I matched the levels from the DNxHD to the Canon and then applied the same levels, contrast, saturation, and a very slight color grading to both clips. No sharpness was added to either.
I did shoot some footage of resolution charts. I'll post pics here at some point. The bottom line is that the hit to the resolution via HDMI appears to be proportional to the scaling, if you scale. Otherwise, it appears to be about full, albeit in a 1620x1080 image. For greenscreen work in which you won't scale the image, this might work well.
It is my expectation (or hope) that the Canon firmware upgrade that has been announced for April 2013 will provide a better HDMI out in that the full 1920x1080 image will be available at "full" (current) resolution. Even better would be if they are able to get 24p out via HDMI to remove the need to deinterlace.
I'm purposefully leaving the video on Vimeo at 720p to show what the footage looks like on a typical web site. You can download the 1080p file, if you wish.