Remote Release From A Slave Unit - Canon ST-E3-RT Instruction Manual

Remote Release from a Slave Unit

You can perform remote release (remote control shooting) from a flash
set as a slave unit. For the operations, see the flash's instruction
manual.
When shooting with this function, the "Release Cable SR-N3" (sold
separately) may be needed, depending on your camera.
Cameras Compatible with Slave Unit Remote Release
For EOS digital cameras released since 2012, such as the EOS-1D X,
the "Release Cable SR-N3" is not needed.
Cameras Not Compatible with Slave Unit Remote Release
Connect the release cable when the power of the camera and the
transmitter is turned off.
Shooting is not possible when focus cannot be achieved with autofocus.
Focusing manually before performing remote release is recommended.
The "Release Cable SR-N3" (sold separately) is for an N3 type remote
control terminal. It cannot be used with cameras equipped with a remote
control terminal other than the N3 type.
Remote release is performed with "Single shooting" regardless of the
camera's drive mode setting.
When there are two or more master units, remote release is performed
using the master unit with the <D> lamp lit in green.
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For EOS cameras other than the above that
are compatible with E-TTL II/E-TTL
autoflash and have an N3 type remote
control terminal, the "Release Cable SR-
N3" (sold separately) is needed to perform
remote release from a slave unit.
As shown in the illustration, use the cable to
connect the camera and the transmitter.