Quote:
Originally Posted by Gidders
Christine
Perhaps I should have said - the remote flashes are definitely triggering but somehow the camera is recording less light even though there is more light being generated. If it was just that the remotes were not triggering, then the histogram should be exactly the same as for just the pop up - but in fact the exposure is less  |
Hi Clive
I think it's a bit more complex than that. My understanding is that, assuming that you are not somehow overriding the ETTL, the camera will attempt to perfectly expose each picture. It does this by measuring the amount of light on the subject and switching the flash(es) off when it calculates that there is enough to generate a good exposure based upon the ISO, aperture and shutter speed.
In other words, all of your histograms should look exactly the same in an ideal world no matter how many flashes you use unless the camera is unable to quench the flashes quickly enough or if, say, one flash at full power was insufficient to light the scene properly.
In the real world however, it is possible that some light from your slave is getting back to the camera directly which the camera assumes has been reflected from the subject and the camera is fooled into thinking that more light has reached the subject than actually has done. It therefore quenches the strobes a fraction earlier than it otherwise would giving a slightly darker image.
The way to compensate for this is to adjust the flash exposure compensation function on your camera.
Does that make sense?
Oh, if you were to take a picture whilst pointing all relevant strobes at a bathroom mirror you should be able to see which strobe(s) are affecting your exposure from the resulting picture.
Sean