The 4x4" Neutral Density (ND) 0.9 Filter has several uses and offers the possibility to achieve otherwise unachievable results. They enable the shooter to adjust exposure without affecting color balance. ND filters appear gray and reduce the amount of light reaching the film.
Determining which neutral density filter yields ideal results for any given lighting situation takes knowledge, experience and a collection of such filters. Speaking generally, the 2-stop value (ND 0.6) effectively compensates average situations.