Hello everyone, been looking for a while but really haven't posted before. My question is about long exposures, 10, 20, 30 seconds or more. How are these figured? I've read comments on some threads like "30 seconds was all I could go without blowing out the sky" or "I tried to expose long enough to get good detail in the <subject> while still leaving the shadow areas dark enough". How are they figuring ahead of time what they need? I am somewhat new to photograpy in that I do well at what I shoot (mostly landscapes) but am very limited on just about everything else. I would like to expand my capabilities as much as possible and can figure out a lot of other types of photography but I have no clue on this one, HELP PLEASE!!!