Canon Speedlite ST-E3-RT Mode D'emploi page 38

q: Wireless Multiple Flash Shooting with Manual Flash Output
Calculating the Shutter Speed
In stroboscopic flash, to ensure that the shutter stays open until the end
of the continuous flashes, set the camera with a shutter speed
calculated with the following equation.
Number of flashes ÷ flash frequency = shutter speed
For example, if the number of flashes is set to 10 (times) and flash
frequency to 5 (Hz), set the shutter speed to 2 sec. or longer.
To avoid degrading and damaging the flash head of the slave unit due to
overheating, do not shoot repeatedly with stroboscopic flash more than
10 times. After shooting 10 times, allow a rest time of at least 15 min.
If you shoot repeatedly more than 10 times, the slave unit's safety
function may activate and restrict the flash firing. If this happens, allow a
rest time of at least 15 min.
Stroboscopic flash is most effective when combining a highly reflective
subject with a dark background.
Using a tripod and remote switch is recommended.
Stroboscopic flash shooting is not possible with 1/1 power or 1/2 power
flash.
Stroboscopic flash shooting is also possible when the camera's shooting
mode is set to "buLb".
When the number of flashes is displayed as "---", flashes are fired
continuously until the shutter closes or the charge runs out. The
maximum number of continuous flashes is shown in the table on the
following page.
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