Hi everybody, I need something explained to me. I'm just learning about expansion and contraction of black and white film during processing for value control, and I am a little confused. I understand that N+1 is an expansion, and N-1 is a contraction, but I don't understand how long the +1 is. If you had a developing time of 10 minutes, is N+1 11 minutes? And N-1 9 minutes? I don't understand if the +1 and -1 are a set amount of time, or if it is a percentage of the original time. If anybody can help me understand, I would appreciate it. Thanks everybody.