Made out of high quality optical glass the lee 150x150mm sw150 mark ii big stopper filter is a long exposure nd filter designed to reduce the amount of light entering the lens by up to 10 f stops compatible with the sw150 mark ii system the big stopper is aimed at landscape photographers who use ultra wide angle lenses by greatly extending exposure times the big stopper has the effect of allowing anything that is moving in your image to become blurred or ghost like eg clouds water etc