The IR AF-assist beam on the Canon Speedlites (I have a 430 EX II) doesn't seem to come on when the DSLR (I have the EOS 7D) is in 'AI Servo' AF-mode. It works fine if I'm in one-shot mode (comes on for a couple of seconds or until focus is confirmed) but it's not used at all in AI Servo mode.
This came up for me at an event recently, where I realised after shooting for a while in servo mode that most of my shots were out of focus, not having realised that it'd disable the AF Assist beam. I've since found a lot of others confused about this, only to eventually realise or be told that it was AI Servo mode preventing its use.
I can't find this limitation documented in the Canon manuals (for the camera or the Speedlite); I can only find it mentioned on forums and posts like the one linked above.
That said, I think it's safe to assume this is an intentional decision by Canon with some justification, rather than an oversight, and would really like to know:
- What is the justification for having no AF Assist in AI Servo mode?
- Are there any custom functions to enable this? Or specific DSLR/Flash combinations or additional accessories (e.g. external power source) that don't suffer this limitation?