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. NDs 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. They are primarily used for light reduction in skylines of landscape shots. Graduated NDs are available in Soft Edge or Hard Edge varieties. Soft edge grads have a smooth transition from clear to ND. Hard edge grads have a more defined hard lined transition and are usually used for longer focal lengths. ND Grads are made in different densities according to the level of light reduction from 1/3 to as much as 4 Stops of light. The 0.9 filter reduces light by 3 Stops. The 165mm size is useable with Hitech 165mm holder systems designed for Wide Angle Lenses. The filter is 165mm Wide but 200mm long to enable it to be easily moved up and down in the holder.