I have just returned from shooting a "first dance" at a wedding where there was very little light on the dance floor.
I set my 1D Mk3 to "P" mode and fired away only to find that 50% of the shots were showing as dark and as though the flash hadn't fired. I held the camera away from me and noted that the flash was definitely firing fully but the result suggested that it maybe wasn't in sync with the shutter.
I then quickly shifted to my trusty 1D Mk2 with same 580EX II flash gun in ETTL mode and got exactly the same random result so it seems its not a camera problem but a flash gun problem.
Never seen this before so any thoughts ?
PS: I changed to a new set of flash gun batteries and it made no difference.