Calling All 580EX Experts!

[photo_photo]

Probably a softball for you guys but...I 'm using a third flash in my Speedlight system and have it as a "C". How do I adjust the Flash Exposure Compensation for this?

[Lotto]

Group C is alway compared to group A and B together in ETTL mode, like you can adjust the group C = (A+B) + 2/3 stop. So if you dial in + 1/3 FEC on the camera or on the master unit and using the same "ratio", then C = (A+B) + 1.

[tim]

Now i've never tried this, but I know group C is treated differently. I remember reading years ago something like that C was for a backlight, or a background light, or something. Have a read of the manual and see what it says. I've only ever used two speedlites in ETTL, more than that and I tend to go manual.

[PacAce]

Probably a softball for you guys but...I 'm using a third flash in my Speedlight system and have it as a "C". How do I adjust the Flash Exposure Compensation for this?

As Tim already indicated, Group C is used for flashes that light the background or provide hair or rim lighting. It is treated a little differently than Group A and Group B in that the Group C lighting is not used to determine flash exposure. So, that being the case, Group C flashes should not be used to light the subject (other than hair or rim lighting) or you will most probably end up overexposing the subject.

As Lotto also indicated, Group C lighting is adjusted relative to the overall lighting of Groups A and B in terms of FEC values. If you want the Group C lighting to be the same as the combined Group A and B lighting, then set the FEC for Group C to 0. If you want it, say, one stop under, set the FEC for Group C to -1 and likewise, if you want it to be one stop more, then set the FEC to +1.

In order to set the FEC for Group C, you need to set the ratio on the master to (A : B) : C instead of A : B which only allows you to set Groups A and B..