Canon Speedlite 600EX II-RT Manual (en)

m Bounce
When the bounce adapter is attached, or when the bounce adapter and 
the wide panel are used together, underexposure may result since the 
flash output decreases. Take necessary countermeasures such as 
increasing ISO speed on the camera or applying flash exposure 
compensation (p.28).
When Quick flash (p.21) is fired with the bounce adapter attached, taking 
the picture after the flash-ready lamp is lit in red is recommended since 
the flash output may not be sufficient.
The flash coverage is set automatically when the bounce adapter is 
attached. You cannot change the setting. (When the <[> button is 
pressed, <BOUNCE ADAPTER> is displayed.)
If you attach the bounce adapter to the flash when using an EOS 
DIGITAL camera released up to 2004, set the white balance to <A>. If 
you shoot with <Q>, appropriate white balance may not be obtained.
The flash light is further softened when the wide panel (p.39) is used 
together with the bounce adapter.
If the subject is dark (underexposed) when you check the shot image, 
perform the flash exposure compensation (p.28). You can also increase 
the ISO speed when using a digital camera.