I'm shooting product and while the color of the actual item is pretty spot on, I noticed the background (which is pure white seamless) is dingy. I'm using: 40D, Alien Bees B800 on right (about 1/16 power), white bounce on left, custom white balance using Whi-Bal card in post. 125 sec, F13, ISO 100 - the item is about a foot from the backdrop. Is there a way to get the background white?? What am I missing here? Thanks!!!