Canon SPEEDLITE EL-1 Advanced User's Manual page 178

Autoflash Photography Using Multiple Receiver Units
When you need more flash output or you want to perform lighting more easily, you can
increase the number of receiver units and fire them as a single flash.
Add a receiver, and follow the same procedure listed in "Automatic flash photography with 1
flash receiver" ( ). You can use any of A, B, or C as the firing group.
When the number of receiver units is increased or sender flash firing is set to ON, automatic
control is performed to fire all flashes at the same flash output and to ensure that the total
flash output results in the standard exposure.
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