Can a Canon 600 EX be used as an on board flash (master) to trigger a 580 II (slave), but as a trigger only... Not actually firing? Or can it also be done the other way around, 580 II on board and 600 as a slave? Sorry for the newbie flash question in advance!
While the 600EX-RT has a new radio system, it also has a fully backward-compatable optical wireless system.
When set not to fire, the Master will still emit command-and-control flashes (and metering preflash when in ETTL mode), but not a flash for exposure.
The final command flash can sometimes be seen in reflective objects in the scene, or if very close to the subject, but there are workarounds for that, too.
