5DIII + 580EX II = Under exposed

This weekend was the first time I tried the 580ex II on the 5DIII. If I was smart, I should have tried it when I got the camera just so I was sure it was working properly. But, I am not smart.

On the 7D, I had no problems so I assumed it would be the same on the new camera. Not so.

Eventually, I figured out that the flash needs to be set with FEC +1 2/3 or else the images are very underexposed.

The flash was set to ETTL and the camera with the 24-105 in manual mode, 125 6.3. There were no mirrors or reflective surfaces to throw it off. Nothing unusual, subjects 10ft away or so.

I have even read through the flash manual (first time for everthing) just to see if there is something I am missing here. I can't really believe the 580EX II doesn't play well with the camera.