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p.1 #6 · White Lightning + 580ex + radiopoppers | |
The red lens is the AF assist light. It has no role in wireless control. The wireless sensor, which is only in play when the flash is in SLAVE mode, is the smaller gray window directly below the flash head.
When used normally you should orient the front of the slave toward the flash head of the Master flash, which as crockett mentioned sends the commands via coded pre-flash, then twist the flash head of the slave toward the subject.
I suspect the reason many people have trouble with Canon wireless is that they are clueless and either block the sensor on the slave with a modifier, or don't orient the base of the slave towards the Master's flash head.
In ETTL ratio mode there is actually a burst of 5-6 discrete command and metering pre-flashes. The master first needs to tell the slave what mode to operate in (ETTL or M).
In wireless M mode the Master tells the slave what power level to operate. Then as the shutter opens it sends the command to fire to the slave. So M mode will also pre-flash in master/slave configuration and should trigger the WL prematurely.
In ETTL A:B ratio mode the A and B groups are pre-flashed separately for ratio setting, then once the necessary power computed in the Master it pre-flashes to tell the groups how to set their power as a % of total capacity. Then A&B are pre-flashed together to meter the exposure.
In ETTL A:B C mode after all the above occurs the Master instructs C - via coded pre-flash - to fire a metering pre-flash. Then C is set relative to A:B so it matches. C is intended only as a background light - it shouldn't hit the foreground because it isn't included in the foreground metering.

All of that happens between the time the shutter is fully pressed and the shutter opens!
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