Neutral Densit (or ND) filters produce a grey neutral effect and are used to reduce light in photography, enabling more control over exposure and depth of field without affecting color or contrast. NDs are made in different densities according to the level of light reduction from 1/3 to as much as 10 stops of light. Prostop filters are a specialist rangeof high density NDs used to create motion blur in long exposure shots and can help create startling effects in water, cloud and other moving objects. The range is available in densities from 6 to 10 stops. Prostops include a foam gasket which sits between the filter and the holder to help eliminate light leakage. Note: if you have a Lee holder system or New Hitech MODULAR holder system a separate Prostop is avaialble with a different size of gasket to fit the Lee and new Hitech Modular holder. The 100mm size is useable with Hitech 100mm holder systems or any 100mm (4") holder. The filtert is also available in 85mm, 150mm and 165mm sizes. Hitech filters are made from the highest quality 1.5mm optical resin.