Canon Speedlite 600EX II-RT Manual (en)

When the camera’s depth-of-field preview button is pressed, the flash 
fires continuously for approx. 1 sec. This feature is called “modeling 
flash”. This is useful for checking shadows cast on the subject by the 
flash light and the lighting balance during wireless flash photography 
(p.57, 89).
Press the depth-of-field preview 
button on the camera.
The flash fires continuously for 
approx. 1 sec.
Modeling FlashN
To avoid degrading and damaging the flash head due to overheating, 
limit modeling flash up to “firing angle 14mm/20mm/24mm: 20 times”, 
“firing angle 28mm: 25 times”, or “firing angle 35mm to 200mm: 30 
times.” After firing modeling flash for the above listed number of times, 
allow a rest time of at least 10 min.
If you fire modeling flash for the above listed number of times, and then 
fire the flash again repeatedly at short intervals, the safety function may 
activate and restrict flash firing. With flash firing restriction level 1, the 
firing interval is automatically set to approx. 8 sec. If this happens, allow 
a rest time of at least 40 min.
During Live View shooting, firing modeling flash (by operating the 
camera) is not possible.
Modeling flash (by operating the camera) is disabled when using the 
flash with EOS M3, EOS M2, EOS M, EOS Elan II/Elan II E/50/50E, EOS 
REBEL 2000/300, EOS REBEL G/500N, EOS REBEL K2/3000V, EOS 
REBEL XS N/REBEL G II/3000N/66, EOS IX, or EOS IX Lite/IX7. Set 
C.Fn-02 to 1 or 2 (p.113), then fire modeling flash using the test flash 
During normal flash photography or when using the flash as the master unit 
in radio transmission/optical transmission wireless shooting, you can fire 
the modeling flash with the test flash button (C.Fn-02/p.113).