Can't use bounce flash with back-lit subjects (5DIII 600ex-rt)

[SwampYankee]

I have a Canon 5DIII and a Canon 600EX-RT and am using a 24-105L. I’m trying to take flash pictures of a backlit subject and bounce the flash of the ceiling. When I do this using full-auto the subject is exposed perfectly. When I try to take the same picture with the P (Program) setting , the subject comes out completely underexposed. I then tried hitting the mn-funtion button because I am told this will expose correctly for the center of the frame (no indication of any setting being saved in the viewfinder). No good, still completely under exposed. I tried sport metering, still under exposed. If I don’t bounce the flash the exposures are consistent between program and auto. I tried an old 580 flash and have the same problem so I know there is nothing wrong with the flash. My Question is why can’t I take pictures of backlit subjects and bounce the flash off the celling. Works great in Auto, terribly backlit of program mode.

 

[rs]

What's the flash exposure compensation set to? Full auto will ignore anything like a big negative FEC setting, instead opting for zero.

Also, what settings does full auto pick (aperture, shutter, ISO), and what is P using? Is it possible full auto is allowing the background to blow out (and therefore letting in enough light from the flash), while P isn't? Knowing what settings the camera chose for you in the two shots will help.

 

[SwampYankee]

NO flash compensation set. New flash, just a couple of days old. The background is not blown out, it's fine, it's just the subject that is under exposed. Full auto nails it, direct flash nails it, bounce flash...underexposed. I am assuming the Program mode will correctly expose whats in the center of the picture but this is not the case

 

[Blakegard]

My recommendation is to shoot Manual exposure with Manual power flash. From the problem you explained it doesn't seem like time is an issue since you took multiple shots testing things out. 99.9% of the time that should fix your problem from my experience.