>Chris/Russ, > Thanks a lot for your valuable advice and suggestions. I >followed them and realized that this was my mistake. I had the >pop-up flash of the D7K in CMD mode but in TTL setting. Once I >changed it to "--", the preflash did not happen from >the camera and everything worked. So final settings... > >Camera pop-up flash: CMD with "--" >SB-700: Group A, TTL >YNU-560: Manual, S1 mode
Unfortunately, I think your YNU may still be firing early, before the shutter opens. You just haven't realized it yet.
When using the --- mode on the Commander, it cancels the Main flash, but it still fires the preflash. The Commander always fires a preflash in all modes.
You have to set the pop-up flash on Manual to eliminate the preflash. But then, you have to put your SB700 in SU-4 mode so it will fire as an optical slave just like your YNU.
To test your setup: Set your camera on rear sync, and then use long shutter speed like one second, you will see the preflash one second before the main flash, because it always fires before the shutter opens. Then you can tell whether or not your setup is working like you think it is.
I thought of one more possibility to explain what you are seeing. The preflash may be too weak to fire your YNU, so, then it would fire only on the main flash. If this is the case, you can't rely on it. In a different placement, the YNU will probably fire on the preflash.