Canon EOS 70D Instruction Manual page 204

Using Wireless FlashN
*1: If the Speedlite does not have a transmission channel setting function, the
camera can work with any channel.
*2: In small rooms, the slave unit may work even if its wireless sensor does not face
the camera. The camera's wireless signals can bounce off the walls and be
received by the slave unit. When using an EX-series Speedlite with fixed flash
head and wireless sensor, take pictures to make sure it can fire.
Canceling the slave unit's auto power off
To cancel the slave unit's auto power off, press the camera's <A>
button. If you are using manual flash firing, press the slave unit's test
firing button to cancel the auto power off.
The camera's master unit function cannot be used for wireless flash
shooting with radio transmission.
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