Here's the challenge: I will (again) be taking photos of children riding horses in a giant outdoor shed. The shed is nearly the size of a football field, has a high roof (15-20 feet maybe), and no sides. The result: the background is the unshaded outside world and, on a sunny day, very, very bright compared to the shaded inside. The answer is flash, of course, but the only one I have now is a Nikon SB-400. I'll be shooting from various distances, often from 10 yards or more, mostly with my Nikon D7000 with an 18-200 lens. Is this flash just too weak for the challenge? What more powerful tool would I need? What can I read that will help me learn more about situations like this? Thank you.