The Lee SW150 Reverse ND 0.9 (3 stop) is a 150x 170mm neutral density filter with a smooth transition from dark to light from the middle of the filter upwards. It is particularly useful for seascapes when shooting bright horizons at sunrise or sunset, & is designed to give a more pleasing & natural balanced exposure to the sky. This filter is also available in 0.6 & 1.2 ND strengths.