There is US patent, not sure if Canon goes this way, but their flash AF assist is not that simple as one might think.Autofocus assist lighting used for rangefinding in very low light conditions.http://www.google.ch/patents/US20100158495
kvbarkley, you might be surprised how complex AF assistant beam is. It is not a simple light.Article is interesting. I wonder if there is IR filter in the front of AF-phase detection sensor? Another interesting observation/speculation.Considering IR ...
First af all this is very annoying problem. I can confirm problem exists.I'm using Canon 1Ds mark III and Canon 600EX-RT flash.Depending on whether flash AF assist beam is on or off I need to set different values for focus micro adjustment. My guess ...