Canon SPEEDLITE EL-1 Advanced User's Manual page 163

Group Control
If you need more flash output or wish to perform more sophisticated lighting, you can
increase the number of receiver units. Simply set an additional receiver unit to the firing
group (A, B, or C) whose flash output you want to increase. There is no restriction on the
number of units.
For example, if you set a firing group with three receiver units to <
treated and controlled as a single firing group A with a large flash output.
Caution
To fire the 3 firing groups A, B and C together, select <
< > setting, firing group C does not fire.
If you shoot with firing group C pointing directly toward the main subject,
overexposure may result.
With certain EOS film cameras that support E-TTL autoflash, you cannot perform
wireless multiple flash shooting with a flash ratio setting.
Note
The flash ratio of 8:1 to 1:1 to 1:8 is equivalent to 3:1 to 1:1 to 1:3 (1/2-stop
increments) when converted to the number of stops.
Firing group A
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>, the three units are
>. With the