Canon EOS 1Ds Mark III Instruction Manual page 108

Flash Photography
Using Non-EX-series Canon Speedlites
With an EZ/E/EG/ML/TL-series Speedlite set in the TTL or A-TTL
autoflash mode, the flash can be fired at full output only. Set the
camera's shooting mode to manual or aperture-priority AE and shoot.
When using a Speedlite which has manual flash mode, shoot in the
manual flash mode.
With an EX-series Speedlite set to TTL autoflash with the flash's
Custom Function, the flash will fire at full output only.
Using Non-Canon Flash Units
Sync Speed
The camera can synchronize with non-Canon compact flash units at
1/250 sec. and slower speeds. With large studio flash units, since the
flash duration is longer, set the sync speed within 1/30 sec. to 1/125
sec. Be sure to test the flash synchronization before shooting.
PC Terminal
If the camera is used with a flash unit or flash accessory dedicated to
another camera brand, the camera may not operate properly and
malfunction may result.
Also, do not connect to the camera's PC terminal any flash unit requiring
250 V or more.
Do not attach a high-voltage flash unit on the camera's hot shoe. It might
not work.
A flash unit attached to the camera's hot shoe and a flash unit connected to
the PC terminal can both be used at the same time.
108
The camera's PC terminal is provided
for flash units having a sync cord. The
PC terminal is threaded to prevent
inadvertent disconnection.
The camera's PC terminal has no
polarity. You can connect any sync
cord regardless of its polarity.