Canon Speedlite ST-E3-RT Mode D'emploi page 64

Troubleshooting Guide
The slave flash does not fire or unexpectedly fires at full output.
Run the channel scan and set the channel with the best radio reception
signal (p.22).
Position the slave unit in clear view of the master unit, without obstacles
between them.
Face the slave unit's front toward the master unit.
The picture is underexposed or overexposed.
If there was a highly reflective object (glass window, etc.) in the picture,
use FE lock (p.30).
If the subject looks very dark or very bright, set flash exposure
compensation (p.27).
When high-speed sync is set, the effective flash range is shorter.
Position the slave unit closer to the subject (p.29).
When using autoflash shooting with three firing groups A, B and C, do
not fire with firing group C pointed toward the main subject (p.33).
When shooting with a different flash mode setting for each firing group,
do not fire with multiple firing groups set to <a> or </>
pointed toward the main subject (p.40).
The picture is very blurred.
When the shooting mode is set to <W> and the scene is dark, slow
sync is enabled automatically (the shutter speed becomes slower). Use
a tripod, or set the shooting mode to <V> or fully automatic mode. Note
that you can also set the sync speed in [Flash sync. speed in Av
mode] (p.49).
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