I have a beauty pageant photoshoot this Saturday. I will be shooting 2-5 graders, and will be using a Canon 580 Speedflash, with either a softbox or a beauty dish. I will be placing the light source straight in front of the subjects. My question is...how far from the subjects should I place the light? I will be using a black backdrop. I will be shooting a 40D with a Tameron 24-70 2.8. Thanks in advance.