Here is the challenge my shooting friends. I have custom function set on both bodies at Cfn 4 - 1.
On my 1D Mark IIn I follow these steps to get an image which is back lit and the subject needs fill flash.
Shooting in AV Mode, Raw, high speed sync turned on the 580, use "*" to focus - then hit the "fel" button to sample the subject for flash. Meter (spot meter) the sky or something 18% gray, recompose and shoot. I have FEC at -2/3 stop (this would be subjective I'm sure). I end up with very good images.
The trouble I'm having is when I use the 5D I don't have a "fel" button so I must disable Cfn 4-1 to Cfn 4-0. When doing this now I must focus and set exposure with the shutter release. So while in Av Mode how in the heck can I expose off the bright sky, focus on the subject, sample flash the subject and get both the sky at a good exposure and the subject....
Curious minds want to know.