Canon EOS 70D Operation Manual - Page 209

Using Wireless FlashN Fully Automatic Shooting with Multiple External Speedlites Multiple Speedlite slave units can be treated as one flash unit or separated into slave groups whose flash ratio can be set. The basic settings are shown below. By changing the [Firing group] setting, you can shoot with various wireless flash setups with multiple Speedlites. Basic settings: Flash mode : E-TTL II Wireless func. :0 Channel : (Same as slave units) [1All] Using multiple slave Speedlites as one flash unit This is convenient when you need a large flash output. All the slave units will fire at the same output and be controlled to obtain a standard exposure. No matter what the slave ID is (A, B, or C), all the slave units will fire as one group. 1 Set [Firing group:1All]. 2 Take the picture. 209