Julie, I just found my post here that has a link to some images that illustrate the dark images at a higher shutter speed.
http://www.nwpphotoforum.com/ubbthreads/showflat.php?Cat=0&Number=6794&an=0&page=2#6794I tested this on Hallie one day in the back yard, it was a sunny day and I shot at f/8, up to f/2.8. The wider the aperture got, the faster the shutter speed and as the shutter speed got faster, each image got darker and darker until at 2.8 it was almost black. I am still not sure why that happens as it makes no sense but I have since been leery to shoot AV for action, I only use it now for portrait stuff. I'd still love to know why this happens though and if I can correct it so I could shoot AV for action too.
