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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:

  1. What is the justification for having no AF Assist in AI Servo mode?
  2. 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?
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up vote 5 down vote accepted

The AF-Beam is is used while the camera focuses but before it meters. Otherwise, metering gets affected and significantly so in condition when the AF-Assist beam may help.

In any type of Continuous AF this is not usually the case, so , the AF-beam must be turned off. A camera must also be able to keep focusing, so the AF-Assist beam could create a situation in which the camera knew where to focus but no longer can, should the AF beam be on but later turned off.

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Ah yeah, hadn't thought of metering. Though I'm not sure why they couldn't implement a time-shared AF/metering mode for AI Servo, rapidly switching between AF-assisted AI Servo and turning off the IR beam for metering. No doubt the performance wouldn't match the fully assisted AF, but it'd have to be better than unassisted AF in the dark. – drfrogsplat Mar 5 '13 at 2:28
It's worth noting that AI Focus also will use the IR beam if you have that mode, but it will also function as One Shot when doing so. I just had to pull out my 600EX to try this out because I hadn't actually noticed this before it was mentioned here. – AJ Henderson Mar 5 '13 at 3:01

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