ghosting from preflash?

apersson850 wrote in post #8216919
the OP never stated that he uses Canon's wireless system. If his gear list in the signature is true to the letter, he has only one 580 EX II.

AFAIK, if the 580EXII is set to master, it will perform a Master's job without knowing whether there are any slaves within communication range.

For example, a windowlight shot may need the background lifting. Put a 580EXII on-camera as Master, with the main flash OFF. In the background, set a 550EX on the ministand, as Group C. Set the Master A:B C ratios to anything.

In this case, the preflash exposure tests from Group A and Group B will tell the camera that there are no such flashes; it does not test-fire Group C. As far as the camera knows, there are no flashes (if indeed it bothers to deduce that). However, when the shutter has opened, the master will still emit a "Fire!" coding and the Group C flash fires.

So, I conclude that if you have set an on-camera flash as master, it will behave as such, regardless.

In E-TTL mode, the master will order group C to pre-flash as well. Otherwise, it can't know what power ratio it's supposed to tell group C to fire with, in order to achieve the FEC you've set for group C.

But you are right in that the master will fire regardless of whether there are slaves responding or not. However, I don't really see the benefit of using it in master mode without slaves?

apersson850 wrote in post #8223701
In E-TTL mode, the master will order group C to pre-flash as well. Otherwise, it can't know what power ratio it's supposed to tell group C to fire with, in order to achieve the FEC you've set for group C.

But you are right in that the master will fire regardless of whether there are slaves responding or not. However, I don't really see the benefit of using it in master mode without slaves?

My recollection was faulty - thanks for the correction.

There is no benefit, but someone learning to use Canon Flashes could have it set as Master because they fiddled. Even the learned might forget previous settings.

That's true, of course. I've done it myself. Took two pictures before I looked at the flash's display and saw what was wrong.

At least the new 580 EX II immediately leave slave mode, if you put them on a camera and do the half-press.

apersson850 wrote in post #8226021
At least the new 580 EX II immediately leave slave mode, if you put them on a camera and do the half-press.

Surprised at this, so tested a 580EXII on my 50D. The flash stayed exactly as set, and the 50D's menu control adopted the settings of the flash. Even when I set the camera to Wireless Mode Disabled, then attached flash to camera, turned both on, and fired a shot, the Master mode remained displayed on the Flash. The same when using FEL button.