Canon EOS-1DX Instruction Manual page 389

The Live View image or captured image is not displayed
during multiple-exposure shooting.
If [On:ContShtng] is set, Live View display, image review after image
capture, and image playback are not possible during shooting (p.186).
The multiple-exposure image is shot in 1 quality.
When the image size is set to 41 or 61, the multiple-exposure
image will be recorded in 1 quality (p.192).
When I use the <f> mode with flash, the shutter speed
becomes slow.
If you shoot at night when the background is dark, the shutter speed
becomes slow automatically (slow-sync shooting) so that both the
subject and background are properly exposed. To prevent a slow
shutter speed, under [z3: External Speedlite control], set [Flash
sync. speed in Av mode] to [1/250-1/60 sec. auto] or [1/250 sec.
(fixed)] (p.200).
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 [z4:
Silent LV shoot.] to [Disable] (p.214).
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.197).
When the [Flash metering mode] flash Custom Function is set to
[TTL] (autoflash), the flash will always be fired at full output (p.204).
Troubleshooting Guide
389