Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 196)

Non-Canon Flash Units
Sync speed
The camera can synchronize with non-Canon compact flash units at up to 1/250 sec.
With large studio flash units, the flash duration is longer than that of a compact flash unit
and varies depending on the model. Before shooting, confirm that flash sync is
performed correctly by taking some test shots at a sync speed of approx. 1/60 sec. to
1/30 sec.
PC terminal
The PC terminal enables you to use flash units with a sync cord. The terminal has
locking threads to prevent accidental cord detachment.
The PC terminal has no polarity. Sync cords can be connected regardless of
polarity.
Caution
Using the camera with a dedicated flash unit or flash accessory for cameras of
other manufacturers poses a risk of malfunction and even damage.
Do not connect flash units with an output voltage of 250 V or more to the camera's
PC terminal.
Do not attach a high-voltage flash unit to the camera's hot shoe. It may not fire.
Note
You can use one flash unit attached to the camera's hot shoe and another
connected to the PC terminal at the same time.
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