None of the Canon cameras turn on the AF assist light (the red light) on external flashes continuously after focus has been achieved.
I’d like to stick a laser pointer (one that projects a pattern) in the flash slot and paint the subject with some sort of laser light pattern to give the AF sensor something to easily focus on. This way, if the laser light is on continuously, then I can use the AI Servo mode to continuously track the subject in low light situations.
In order to turn on/off the laser pointer, I will use the top left most small pin off the flash hot shoe (not the big round one). See diagram in this thread.
Canon hotshoe pinout description here.
I may have to tweak the diagram shown in the link to turn on the laser pointer, but it should be doable.
The beauty of using the signal off the hot shoe connector is that it comes on only when the camera needs to focus, and stays on only for 7 seconds after it’s done focusing, or 3 sec after the shutter is fired.
Update: 20 Sept 2010
I went to Club neutral last night and shot some pictures there. (Description of results from that night’s shooting at this post) I took the laser pointer with me, but I could not reliably use it. I had no mount for it other then the velcro I brought with me, which couldn’t keep it in one place especially when having to push the button all the time to trigger it.
When leaving the club noticed a red led bike light, and thought it may work for me too. Tried the red led bike light I have, to see how well it would assist the camera in focusing. Compared to the AF assist light from the Sigma EF500 DG super flash, it took almost twice as long to focus, and at times the bike light didn’t even acquire focus. So much for a simple solution.
UPDATE: 24 Sept 2010
Came across a few forum posts where people are discussing AF assist lights. This chinese site has an interesting idea.
UPDATE: 02 May 2011
I have progressed this project further. Check out this post for a working laser AF assist tool.