Canon EOS 5DS Instruction Manual page 478

Troubleshooting Guide
The flash does not fire.
Make sure the flash (or PC sync cord) is securely attached to the
camera.
If you use a non-Canon flash unit with Live View shooting, set [z6:
Silent LV shoot.] to [Disable] (p.266).
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.245).
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.253).
Flash exposure compensation cannot be set.
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.
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.248).
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.257).
478