Canon ST-E3-RT-V2 Manual page 30

Using Fully Automatic Wireless Flash
r Second-Curtain Sync
Shooting with a slow shutter speed and second-curtain sync captures
the trail of the light sources of a moving subject, such as car lights, in a
natural way. The flash fires right before the exposure finishes (shutter
closes).
Check the cameras that support second-curtain sync on the Canon

website.
The second-curtain sync works well when the camera's shooting mode is

set to < > (bulb shooting).
5
When the flash mode is set to <a>, the flash fires twice. The first

flash is a preflash to determine the flash output. It is not a malfunction.
To return to normal flash shooting, press function button 2 <Y> to

turn off <r>.
30
1
Display <P>.
Press function button 4 to display

<P>.
2
Set the sync setting to <r>.
Press function button <Y> to set

<r>.