Hi all. I've been considering doing some ND photography. It works well with architecture (a favorite subject) as the long exposures allow me to get a good image of a building as people move through without leaving much of an impression. Can I get some guidance on NDs from anyone here who already has some experience? How dense? How do I gauge density vs. what I'm trying to accomplish. I assume this will be more than just a couple of f-stops. Is there a particular brand that has struck any of you as particularly avoidable or recommendable?