under exposure issues using flash on 5d mk3 with 600 ex and 580 ex2 need to set exposure compensatio

[pentax000]

Body 5D mk3,

Normal expososures are pretty good i don't have to set compensation most of the time.

Using flash i need to set extra compensation ,usually 0.7 to 1.5 to get a correct exposure.

I have a 580ex II and a 600ex-rt same results

any ideas peeps

[RobertTheFat]

Canon's E-TTL protocol is highly averse to blown highlights and, in effect, takes the position that all flash should be fill. So it exposes for the highlights and relies mostly on ambient light for the background. This works well if there is enough ambient light. It also works pretty well for bounce flash, since the flash unit creates its own ambient light. But outdoors at night - you've gotta set the flash to manual.

[cicopo]

Not sure which compensation you're using but you have Exposure Compensation AND Flash Exposure Compensation available and between them you should be able to light up most scenes unless it's a very large hall etc. When you run up against that you have to up the ISO too.

[Skirball]

There are far too many variables that you don't discuss for any of us to be able to give any sort of useable information.  Using a camera in an automatic setting gives plenty of variables, but add in two flashes, at least one of them off camera, potentially eTTL...  who can say. 

I don't recommend shooting in automatic camera modes with automatic flash (TTL), unless you're ok just letting the camera figure it all out.