Canon slave question...

Odd one here...

I was playing with my 580EXII and another 580EXII t'other day. One on-camera, one off-camera. The on-camera flash was set 'not firing' - so just the slave fired.

I set it all up to knock out the ambient light, indoors. I think I was at something like 1/200th, f/6.3 and ISO100 and flash was set to 1/16th power.

Now, the batteries in the off-camera 580EXII were duds, which I didn't know. So I was taking shots at first wondering why the off-camera flash wasn't firing. I then changed the batteries and carried on with what I was doing, and it was all good.

The thing is that later, In Lightroom, the shots that were taken at first with the flat batteries where the off-camera flash wasn't firing, had slight shadowing from the pre-flash from the master firing the 'signal' to the off-camera flash, and this showed up in the photos and looked like a very weak flash hitting the subject.

Is this normal? I'm now thinking that this weak flash (from the master) may show up in shots that I may do in future, where the slaved flash(es) are set to a low-power? Does that make sense?

sando wrote in post #6292837
The thing is that later, In Lightroom, the shots that were taken at first with the flat batteries where the off-camera flash wasn't firing, had slight shadowing from the pre-flash from the master firing the 'signal' to the off-camera flash, and this showed up in the photos and looked like a very weak flash hitting the subject.

Is this normal? I'm now thinking that this weak flash (from the master) may show up in shots that I may do in future, where the slaved flash(es) are set to a low-power? Does that make sense?

Yes, this is normal. Just before the master fires (or doesn't fire), it sends a quick pulse to trigger the remote flashes. This trigger pulse happens when your shutter is already open, so you will see it in your images if you shoot at high ISO and large apertures.

If this is a show stopper for your pictures, use the ST-E2 instead, which transmits mostly in the IR band.