600ex-rt shooting into another flash kills exposure

Hey all, I recently upgraded all our flashes to 600's and am now having an issue I can't sort out. Our standard setup at a reception is to have 1-2 flashes around the dance floor and one on camera for fill. In the past I could put the OFC's on manual and the on camera flash on ETTL and everything was good. I could shoot right into the OFC if I wanted and the on camera flash still did it's ETTL thing and filled correctly.

With the 600's this isn't the case. If I clip an OFC at all, as in any part of it's light hits the frame, it kills my on camera flash output and results in an underexposed rimlit image. So whre I used to purposely get a few images with the OFC in it, or bursts of light between the couple, etc. now I have to constantly watch out for the OFC and make sure I don't get any part of it, or it's spill, in my frame.

Any input on why this is, what has changed, and how I can correct it?

FYI - my old setup was 580ex's with Yongnuo ETTL triggers.

Interesting. I have the same setup, 2x 600 OCF in M and 2x600 in ETTL on camera. I didn't pay attention much but I'll test and see if I also have the same issue. I shoot cross light alot and if there was any OCF pre-flash I would have seen it.

Love to see other's thoughts too.

Best to setup your lights in Gr mode.
I do it all the time without a problem.
Gr A is on the camera, Gr B is one off camera light, Gr C is the second. Its possible to have up to 5 groups but I generally stay with three.

Now I can control each group as ETTL using FEC setting per group. Or Manual and set power per group.

I use ETTL and setup a bit of a ratio between B and C, then use A for fill.

What's nice is that the ratio stays constant regardless where I am on the dance floor.

This was shot using that technique. Two lights, one on either side of stage as kickers. One with yellow gel the other with red gel. Group A light was on a stand camera left in front of stage about 20 feet back. STE3 on camera to fire them.

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Sooo I got out the manual, my lights and camera and went to work trying to figure this out after reading the responses. Turns out dmward was absolutely correct. What I didn't know/understand is that when the flash on camera is set to fire it is ALWAYS assigned group A. I was using group mode, so I had that right, but I was putting my OCF in Group A if 1 or A & B if 2. I guess I thought the on camera flash was in some magical "on camera" group of it's own. Nope.

I did some testing, and putting the OCF in group A got horrible results just like I have been seeing. Very inconsistent and dark if the OCF was in frame. Putting the OCF in group B returned the consistent results that I have been expecting and the on camera flash nailed the fill every time.

Thanks a bunch guys. Frustrating problem solved!

The master being in a group of its own can happen only when you use the older optical master/slave system and use a built-in flash as the master. Like what you can do with a 7D and slaves like the 580 EX II or the 600 EX-RT, when it's set to optical slave mode.