There is discussion on another thread that suggests the Yongnuo YN565 spits out an AF Assist beam that makes the 5D3 AF more reliable at very low light venues than the Canon 580EXII & 600EX-RT. One poster put up images of the bright Yongnuo AF Assist beam projected on his ceiling, and said that with the Yongnuo YN565 his 7D will focus in pitch darkness. This is interesting enough to look into a bit further. My guess is that it depends a little on what you're trying to focus on. Any real-world feedback on this?
Like others I have been professionally embarrassed by the glacial speed of getting an AF lock with 5D3 & 580EXII at low light venues. If the Yongnuo YN565 does in fact offer a solution, I'll get a couple of them just for extreme low light functions. The YN565 has a side benefit of being able to accept external power sources. Bizarrely, the newer YN568EX which supports HSS doesn't have a plug for external batteries. Go figure...
I got one of the recent bigvalue 5D Mark III cameras (moved up from a 60D, so I'm still trying to digest the manual), and around the same time bought a pair of YN622C triggers and a YN568EX flash specifically for shooting in very dark bars and venues in and around NYC. I was getting a little worried reading about all the AF problems.
I can definitely confirm the AF assist the 622C on top of the camera puts out has me achieving focus lock (turned on my focus beeps so I could hear it) in a near pitch black room in a second or less with a 50mm II 1.8 lens. My Tamron 24 - 70 2.8 VC gives a similar response, maybe just a tad slower than my nifty. The AF pattern seems to be similar to a YN565 flash af assist I saw on another thread (a pattern of vertical and horizontal stripes within a circular area).
Attaching just the YN568EX as on camera flash netted similar results, though it was a little slower than the NY622C. Still, longest time to focus beep was a little over 1 second in a near pitch black room. It has the same kind of patterning - a collection of vertical and horizontal stripes within a circular area).
This was done using center point AF on the camera. If anyone wants to know how a particular AF point (and\or point mode) responds on the 622 or the 568, let me know.