Having just noticed one of the POWs (Morning Light by Dave K) was done using a ND 2 Singh-Ray Galen Rowell filter, I set about finding out what they are. Well, I've found them in several places, and so far I'm clueless as to how one uses them. Are they hand held in front of the lens? Is there some sort of bracket or holder one can use? I appreciate any enlightenment you can give this learner!