I purchased a 5D mkIII back in April as an upgrade to my 7D, I've been very happy with the new camera until the other evening when I was shooting in some low light situations with the flash.
For reasons that seem to be eluding me I can't find the setting in camera to activate the focus assist beam from the flash. It fires with the proper exposures though I have to switch to live view for my auto focus to lock on.
When the same flash is on the 7D this is not an issue, the auto focus assist beam kicks in as needed .
Has anyone else had this problem with a 5Dmk III ?
I don't have my camera with me at the moment, but I seem to recall some confusion about how Canon labels the setting.
I don't believe they have a setting which claims it enables or disables the focus assist beam... I think they have a setting whose label says it will Enable/Disable firing of the flash.
What the setting _really_ does is allows you to use the focus assist beam without having the flash fire. I think the focus assist beam is automatically enabled.
If you can't get a specific answer, I'll grab my 5D III & 580EX II and verify this.
On the 5D III go into:
Menu -> SHOOT1 (Red Camera 1st page) -> External Speedlite Control -> Flash C.Fn settings -> Setting #8 (AF-assist beam firing) -> 0 (Enabled)
Just verified it with my 5D III / 580EX II and this does enable/disable on mine.
Yeah, that explains it... Flash Focus Assist doesn't work on 7D or 5DII in AI Servo, either. It makes sense if you think about it. AI Servo doesn't have Focus Confirmation or lock the way One Shot does. AI Servo is continous to track movement, so it would need continuous Flash Focus Assist beam to work and that would drain the flash batteries pretty rapidly.
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