There seem to be many people asking simple-sounding questions (i.e. regarding grey cards, light meters etc.) for which the answers can be exceedingly complex and abstruse. A number of very knowledgeable people have been posting answers, or attempting to answer these questions. Without denigrating anyone I would suggest it is vital to be familiar with the basic tools and principles of photography before getting tangled up in grey card 12% vs 18% readings, "K" factors, wherein I am entangled at the moment etc. etc. Reference to a good, basic text on photography will, I think, clarify much that goes on here. I keep making reference to a book by John Upton and, I think, Barbara London only because I think it one of many excellent texts. While I greatly enjoy and am learning much from many of those posting on this thread I fear it will confuse many with too much good information. I for one am still trying to digest Stephen Benskin.s posts even though I have read fairly widely. Another book, quite comprehensive, is Leslie Stroebel's"View Camera Technique". Much more than elementary but very well illustrated. Well worth the time and money!