I just did some portraits, and im wishing I went ahead and got a card that helps with white ballance. Problem is I dont remember much about those. Whats the best one to buy? And where? And how do you use them? I believe you simply have the subject hold the card that has a white strip, black strip and 18% grey strip, and then do the rest of the shots right after with out chainging the light. Then in photoshop you use the first picture to sample the 3 things in curves. But I could be mistaken. Any help would be appreciated.