The neutral gray half of this filter transmits 50% of the incoming light, so that it darkens the respective portion of the subject by one f-stop without altering its colors. For example, when the sky is too bright in relation to the landscape, this is an ideal amount for good detail rendition in the clouds and for preventing the sky from being washed out" by over-exposure. Note The (701M) and (702M) series of filters are supplied in a rotating mount (except 4x6 filters), similar to that of a polarizer. Made of optical glass with multi-resistant coating (MRC). Multi-Resistant Coating (MRC) with Nano Technology The nano coating is an outer layer of protection that comes standard with all XS-Pro Digital MRC filters. The nanotechnology based characteristic (lotus effect) produces a better beading effect with water making the cleaning of this filter even simpler and faster than ever before. MRC nano has an improved outer (8th) layer over regular MRC. MRC by B+W is not only an extraordinarily effective multiple layer coating, it is also harder than glass, so that it protects filters from scratches (for instance when cleaning the filters) and it is also water and dirt repellent, thus facilitating filter maintenance.