580exII 430exII 5DmkIII focus assist issue

Hey all,
Been mucking around with my flashes, re-reading through manuals etc. I really don't use them very much.

Think I might have an issue with the focus assist. If I have the flash on the camera and set flash firing to disable, the flash should emit a focus beam should it not? Neither the 580ex2 nor 430ex2 will emit the focus beam.

Anyone have any thoughts as to what settings I may have missed? Or is it possibly an issue with the body?

Are you using AI focus mode on the camera? Focus assist is disabled in that mode.

Oh wow, thanks Dale. Yes I do. I use the back button focusing so I have both worlds in the one button.
Going back to read the manual again, that bit obviously didn't sink in.

I use the same setup. Want to guess how I know this does not work?

You've piqued my interest now. Do tell.