Canon EOS-1D C Instruction Manual page 381

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.198).
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.212).
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.195).
When the [Flash metering mode] flash Custom Function is set to
[TTL] (autoflash), the flash will always be fired at full output (p.202).
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 [z3: External Speedlite control], set [Flash sync. speed in
Av mode] to [Auto] (p.198).
Troubleshooting Guide
381