Canon Speedlite 600EX II-RT Manual (en)

?: Stroboscopic FlashN
To ensure that the shutter stays open until the end of the continuous 
flashes for stroboscopic flash, 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.
Calculating the Shutter Speed
To avoid degrading and damaging the flash head due to overheating, set 
repeated shooting with stroboscopic flash to 30 times or less. After 
shooting 30 times, allow a rest time of at least 10 min.
If you shoot repeatedly more than 30 times, the safety function may 
activate and restrict flash firing. If this happens, allow a rest time of at 
least 40 min.
When performing stroboscopic flash, combining a highly reflective 
subject with a dark background is most effective.
Using a tripod, remote switch, and Compact Battery Pack CP-E4N (sold 
separately/p.123) is recommended.
Neither 1/1 power nor 1/2 power flash can be set.
You can also perform stroboscopic flash even when the camera’s 
shooting mode is set to <bulb(B)> (bulb shooting).
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 next 
High-speed sync (p.31) cannot be set during stroboscopic flash.