Master flash disabled , only slave firing --> why preflash req?

this could be a really basic question. i've always used generic flash with manual output modes and i have never owned two ETTL flashes until now so i'm playing around with ETTL master/slaves.

i have 580EXII as master, controlling a slave 430EX.

if in a case where i just want slave to fire, i disable master 580EX via disabling master per page 39 in the manual. but the master unit still fires... i dont get why?

it says in a small note in the page 39 that "it will still preflash to transmit wireless signals" to slave but i thought that was what the IR was for? so why does it need to send preflash?

i don't have a setup like this, but my guess is to get ETTL for the slave the preflash is required...

It's not really IR, but semi-IR or near-IR. Which is what the ST-E2 master controller uses, but the 580EXII uses visible light (in multiple morse code like pulses fired at very high speed) that is filtered to near-IR by the slave unit's window.

If you like, in theory you can dim them down to near-IR with a filter but nobody bothers because they all go out before the shutter actually opens so don't effect the exposure.

thanks for the info. so everything is working ok. surprisingly this wireless system works REALLY well. i thought it was line of site stuff so i always used cheap radio triggers for my sunpak manual flashes... but i finally collected two canon flashes and started working with these and i'm impressed.