Canon Speedlite EL-100 - Manual (Page 43)

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Optical Transmission Wireless Flash Photography
Wireless Multiple-Flash Photography
E-TTL II/E-TTL autoflash photography is possible with two or three
receiver groups, and the flash ratio (proportion of flash output) can be
adjusted as needed.
E-TTL II/E-TTL autoflash photography is also possible with all flash
units firing at the same output level (p.47).
z
Autoflash with receiver groups
B
A
B
A
C
2 groups (A, B) (p.49) 3 groups (A, B, C) (p.50)