some ?? on flash photography

[davidreeves]

I recently purchased a Sigma EF500 ST DG for my Canon Digital Rebel XT and I also bought a StoFen diffuser.

Most of my shooting is HS and College basketball games. I somehow got roped into taking individual and team pictures of a little girls basketball team, portrait shots. I am really new with flash photography and would appreciate someone sharing their wisdom with me.

I grasp the bounce flash concept and have taken a few practice shots in the place I will be shooting.

It seems like most of the time when I use the Sigma flash, I wind up with serious overexposures. The attatched pic is an example. It was taken in a meeting room, (the one I plan to use to take the team pictures) no outside lighting, fluroscent lighting, Sigma 28-70 f2.8 lena at 70mm, the sigma flash (ef500 dg st) with diffuser and the flash pointed straight up at the ceiling which is white ceiling tile and the ceiling height is about 10 feet. I was about 20 feet from the subject in this picture. Adjusting the flash head downward slightly increases the overexposure problem.

Use of the diffuser also seems to increase the over exposure issue and I thought it would do the opposite by providing more even lighting. I do have the flash compensation setting on the camera set to underexpose slightly and the flash itself is set for TTL

All that said can someone give me some rule of thumb guidelines? It there a way to tone down the flash strength?

Would appreciate any comments.

Thanks.