TTL Underexposure (600EX) when shooting directly into another strobe

[BianchiOltre]

Hey guys, I love shooting with three Canon 600EX stobes, two on light stands, and the trigger unit on camera. This works great unless one or both of the strobes are in frame that fool the sensor into causing massive drop in light output from the main unit. Is there a way to avoid this? My only solution so far is try to block the two lights on stands with a subject in the foreground. Thanks!

[kvbarkley]

You might have to shoot manually.

Are you saying that the strobe is in the frame firing directly at the camera?

[BianchiOltre]

yes

[rs-eos]

You'll have to use manual on the flash that's on/alongside your camera.   When using ETTL, a pre-flash will be fired (approx 1/32 power).  That is then used to compute what the flash power should be for your scene.   During this period, if two flashes on ETTL "see" each other, they will greatly dial down the power since the scene is too bright.

Using manual solves this since no pre-flash fires for that flash.  i.e. the other flashes will no longer see it during the pre-flash phase.

[BianchiOltre]

Thats what I suspected but hoped for a different answer.