The Grad Kit 3 contains a complete set of the Reverse Grad Neutral Density filter – 0.3 (1 stop), 0.6 (2 stops) and 0.9 (3 stops) to maximise the effectiveness of your sunrise and sunset photography. Neutral density (or ND) filters produce a grey neutral tone and are used in photography to reduce light, enabling more control over exposure and depth of field without affecting color contrast. ND's are also available in graduated filters, both in hard and soft edge. These filters are clear in the bottom half of the filter with a gradual transition to the ND effect in the top half. The reverse grad is a vital tool if you are looking to take sunrise or sunset shots. This is because the ND part of the grad is upside down, with the darker part at the middle of the filter and the lighter part at the top. This ensures the maximum light reduction when the sun is at or near the horizon. This 150mm filter kit is useable with Lee 150mm holder systems and similar holders. The graduated filters in this kit are 150mm wide but 170mm long to enable easy movement up and down in the holder. The kit is also available in 85mm, 100mm and 165mm sizes. Hitech filters are made from the highest quality 1.5mm optical resin.