During an upcoming trip to Michigan, I'd like to capture some low-angle, wide, long exposures on the short. I'd like to achieve an exposure of between 7 - 20 seconds ideally 30 minutes before and after sunrise/set. Examples: http://us.123rf.com …radise-island-on-beac.jpg, http://www.davidroossien.com …gLakeMichigan_730x497.jpg
How many stops should I go? I've heard that anything over 3 does goofy things to the image and that stacking even two lenses causes vignetting. I also haven't heard good things about variable ND's.
I'm shooting with a 40D and I'll be using my 18-200mm (72mm filter) and possibly my Sigma 150mm Macro. I also have a 100-400L (77mm filter). I plan to purchase the 72mm filters and will buy separately for the 100-400 as needed.