why not get a LCW faderND... with your lens being on the wide side this should take care of most of your needs. There are people showing that the cheaper faderND's have too much softening @ longer focal lengths, but 10-20, you should be golden. Also if you are not using a Full Frame sensor, then I wouldn't worry too much about vingetting from filters, I run a 46mm faderND on my 20mm f1.6 46mm thread lens and I get no vingette. I would worry more on vingetting when you start using a lot of step up rings to use larger filters on smaller thread lenses.