Flash Syncing

[debwesleyphoto]

Hi im new to the forum and have a new question about flash syncing. Im not sure if this is the way ro ask the question. 

The scenario I experience while shooting a wedding with one 600 ex rt flash acting as the master on camera (and actively flashing), and the other acting as a slave on a light stand. When in ettl mode, the flashes are both firiing but not at the same time. The slave flash is delayed, so therefore not contribting to the image. If I put the master in manual mode, it syncs fine with the slave. How can this be corrected?

[kvbarkley]

Are you using radio or optical mode? If using optical mode you might be seeing the pre-flash from the master, which happens right before both fire for the exposure making it seem like there is a delay.

[RobertTheFat]

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@kvbarkley wrote:

Are you using radio or optical mode? If using optical mode you might be seeing the pre-flash from the master, which happens right before both fire for the exposure making it seem like there is a delay.

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It even occurred to me to wonder whether the master was set to radio and the slave to optical. Conceivably the slave could then misinterpret the master's main flash as a pre-flash and therefore fire late. I doubt that it's that simple, but it might at least be worth ruling out.