Canon EOS 5D Mark III Instruction Manual page 369

The flash always fires at full output.
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).
Flash exposure compensation cannot be set.
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
camera.
High-speed sync cannot be set in the <f> mode.
Under [z1: External Speedlite control], set [Flash sync. speed in
Av mode] to [Auto] (p.194).
The shutter makes two shooting sounds during
Live View shooting.
If you use flash, the shutter will make two sounds each time you shoot
(p.201).
During Live View and movie shooting, a white
icon is displayed.
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).
Troubleshooting Guide
or red E
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