Complex sentence: In photography and optics, a neutral density filter or ND filter is a "grey" filter; it reduces light of all wavelengths or colors equally, much like inexpensive sunglasses.
Split into simpler sentences: In photography and optics, a neutral density filter or ND filter is a "grey" filter;. In theory, a neutral density filter reduces light of all wavelengths or colors equally, much like inexpensive sunglasses.