Canon EOS 5D MARK III Instruction Manual page 90

Flash Photography
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"_P _ If the camera is used with a flash unit or flash accessory dedicated to
another camera branc the camera may not operate properly and
malfunction may result.
Do not connect to the camera's PC terminal any flash unit requirin 9
250 V or more.
® Do not attach a high-voltage flash unit on the camera's hot shoe. It may
not fire.
] 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.
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