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 usuallye 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. This 85mm Hard Edge filter is usable with Hitech 85mm holder systems or any 85mm (3.35"/P Size) holder. The 0.9 filter reduces light by 3 stops. The filter is 85mm wide but 110mm (4.35") long so that it can be moved up and down in the holder to line up the graduation line easily with the horizon. The filter is also available in 100mm, 150mm and 165mm sizes. Hitech filters are made from the highest quality 1.5mm optical resin.