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. These filters are clear in the bottom half of the filter with a gradual transition to the ND effect in the top half. They are primarily used for light reduction in skylines of landscape shots. 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 100mm filter is usable with Formatt-Hitech 100mm holder systems or any similar 100mm holder.