I agree with Jim, shoot in manual... any automatic exposure mode will meter for ambient which may not be what you want
However the main issue here is your Stofen - It may diffuse the light, but at the same time it would cut down the GN of the flash greatly for group shot. You may want to add another 550/580 as a slave to increase the output, or use some bigger device like a QFlash, or similar device for larger group
-> Especially with multiple rows setting, a weak flash will decay as it extend further back, and you may have to use smaller apertures as well to keep everyone in focus...
Last edited by NHL; Mar 26, 2010 at 9:30 AM.