I have decided to do contact prints of a bunch of my negatives, I started with the 4x5s and they printed out nicely. But when I was printing some of my earlier negatives (which scanned really nicely), It seems that the contact prints are really dark, and some are almost black. I contact print using a 7w bulb, suspended about 3 feet above the paper and negatives and exposure times are around 15-20secs. Do I need to increase exposure times to get a usable pictures out of the "thin" negatives? Thanks