I'm doing portrait work for a high school marching band next week, which involves a group shot of about 100 kids in bleachers outside. I went to the stadium this morning to check things out, and have realized that if there is sun, it will be behind the bleachers. Obviously I'll be too far away to utilize any kind of flash. I'll be shooting with a Rebel XTI with a 70-200mm f2.8 lens perched on top of a 12 foot ladder. Any suggestions on dealing with back light would be great!