>Thanks for the follow up Russ. So, I tried my tests again >and did indeed confirm that the YNU was firing correctly. >Here's what I actually see as the sequence.. > >i. Pre-flash fires from camera, pop up flash >ii. YNU flash fires during pre-flash >iii. Wait for 2 seconds >iv. YNU flash fires again at the end of the rear curtain. >i'm using the YNU-560 flash, if it makes any difference. > >Also, if I change the pop up flash from "--" to >"M", the same behavior occurs.
Now I think I understand what is happening.
You must have the YNU set to a fairly low power, so that it fires on the preflash, and it is recycling just enough to fire again on the main flash. I think you will find that if you shorten the shutter, and/or increase the power of the YNU, it will fire on the preflash, but will not have time to recycle, so it will not flash again on the main flash.