As we all know, Canon's ETTL2 flash system only allows you to control 3 groups of flash (A, B, C). You can independently set ratios between these flash units (A:B and A:C). What if you want to add a fourth group of flash? I was curious to see if you could fake a fourth group by assigning it to group C, and then set FEC on the slave unit to get a different output from that particular slave unit.
I could not find any answers anywhere on the internet so I decided to do my own experiment.
Method. The test subject (a wonderful example of communist kitsch!) was placed on my loudspeaker. A 580EX was mounted on the camera (EOS-20D, manual mode, 1/100s, ISO100, tripod mounted). A 50/1.4 lens was used, set at F/8. The 580EX was angled so that it would bounce off the wall to the left. A 550EX slave was assigned to group B and placed to the right of the picture. Both flashes were left in ETTL mode. During the experiment, the only variables were the RATIO and FEC settings.
Results
First, I did some test shots with settings which are known to work just to prove that my setup will deliver the experimental results which I am looking for.
