Why does the IR-Assist beam of Canon Speedlites not get used in AI Servo AF mode?

Question

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?

Answer 1

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.

Answer 2

In addition to the metering issues well addressed in Itai's answer, there is also the design limitation of AI Servo compared to One Shot in the type of dim light that the IR AF Assist Beam is needed.

When in AI Servo the pixels in the PDAF sensor array get a shorter "exposure" time than they do in One Shot. In very dim light the signal to noise ratio on the data from the PDAF sensor makes the information indistinguishable from random noise. The shorter the "exposure" time for the PDAF sensor, the lower the S/N ratio. This means AI Servo will not focus in lighting conditions as dim as in which One Shot is capable of focusing. These are the type of conditions when IR AF Assist is needed, so it only makes sense that IR AF Assist would only work with One Shot which is more suited to focusing in extremely low light.

Chuck Westfall (Chief Technical Advisor for Canon USA Consumer Imaging Group) has said in the past there about two stops difference in sensitivity between AI Servo and One Shot at the center point for the models current at that time (2014). More recently he has declined to answer such questions with that level of specific information. Many have theorized that Canon Japan has instructed him to reveal less information when that information could be used to deduce technical trade secrets.