Fotodiox Pro 6.6x8.6" filters are the strongest in the industry at 4mm thick. Fotodiox 6.6x8.6" filters are practically unbreakable and are guaranteed spectrally neutral to within + or ? 3 in the a and b channels of the lab color space. Fotodiox Pro filters are also guaranteed to be within + or ? 2 in regards to total light transmission. Fotodiox currently offers 6.6x8.6” in 4 Graduated Neutral Density (ND) filters: .6 Soft Edge, .6 Hard Edge, .9 Soft Edge and .9 Hard Edge.