Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 512)

Troubleshooting Guide
512
If you use a flash unit other than an EX-series Speedlite, the flash will
always be fired at full output (p.261).
When the flash Custom Function setting for [ Flash metering mode ] is
set to [ TTL flash metering ] (autoflash), the flash will always be fired at
full output (p.268).
If flash exposure compensation is already set with the Speedlite, flash
exposure compensation cannot be set with the camera. When the
external Speedlite’s flash exposure compensation is canceled (set to
0), flash exposure compensation can be set with the camera.
Under [ z 3: External Speedlite control ], set [ Flash sync. speed in
Av mode ] to [ Auto ] (p.264).
If you use flash, the shutter will make two release sounds each time
you shoot (p.274).
It indicates that the camera’s internal temperature is high. If the white
<
s > icon is displayed, the still photo’s image quality may deteriorate.
If the red < E > icon is displayed, it indicates that the Live View
shooting will soon stop automatically (p.293).
If the image size is set to 41 or 61 , the shooting images are not
displayed during continuous shooting (p.273).
The flash always fires at full output.
Flash exposure compensation cannot be set.
High-speed sync cannot be set in the mode.
The shutter makes two release sounds during Live View shooting.
During Live View shooting, a white < s > or red < E > icon is displayed.
Shot images are not displayed when continuous shooting is
performed during Live View shooting.