There are varying degrees of ND filters from 2 stops up to 8+ stops. With a 2-stop (0.6) ND filter you'll lose two stops; on your M9 if you can only shoot at f/2.8 and 1/4000th of a second (at ISO 160), then the use of a 0.6 ND filter would allow you to shoot without overexposure at f/1.4 and 1/4000th of a second. A 3-Stop or 0.9 ND filter would do the same with a 3 stop difference (so f/1.4 and 1/2000th). The practical reason for the creation of an ND filter is for us film users that can't change the ISO of our film mid-roll, often forcing us to shoot everything at f/16 and 1/1000th of a sec on a very sunny day with something like Tri-X 400 in the camera.
I'd also suggest you get a B+W MRC ND filter to avoid degradation to your images or potential flare, and probably the 0.9 ND (3 stop) would be a good choice (though the 0.6 might be enough in 90% of your situations) to start with...
_________________ ;Alex Flickr Photostream
|