Canon EOS 5D Mark III Instruction Manual (en)

Troubleshooting Guide
If you use a flash unit other than an EX-series Speedlite, the flash will 
always be fired at full output (p.191).
When the [Flash metering mode] flash Custom Function is set to 
[TTL (autoflash)], the flash will always be fired at full output (p.198).
If flash exposure compensation has already been set with the 
Speedlite, flash exposure compensation cannot be set with the 
camera. When the Speedlite’s flash exposure compensation is 
canceled (set to 0), flash exposure compensation can be set with the 
Under [z1: External Speedlite control], set [Flash sync. speed in 
Av mode
] to [Auto] (p.194).
If you use flash, the shutter will make two sounds each time you shoot 
It indicates that the camera’s internal temperature is high. If the white 
> icon displayed, the still photo’s image quality may deteriorate. If 
the red <E> icon is displayed, it indicates that the Live View or movie 
shooting will soon be terminated automatically (p.217, 247).
The flash always fires at full output.
Flash exposure compensation cannot be set.
High-speed sync cannot be set in the <f> mode.
The shutter makes two shooting sounds during 
Live View shooting.
During Live View and movie shooting, a white   or red E 
icon is displayed.