SPECIAL SIZE FILTER MADE TO FIT FORMATT HITECH MODULAR AND LEE 100mm HOLDER SYSTEMS. Neutral Density (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 range of 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 1 to 10 stops. Prostops include a foam gasket which sits between the filter and the holder to help eliminate light leakage. The gasket on this filter has been made in a size to enable the filter to fit Formatt Hitech Modular and Lee 100mm holder systems. It should only be purchased if you have a Formatt Hitech Modular or Lee holder system. A filter with a different size gasket is available for other systems. The 100mm size is useable with Formatt Hitech 100mm holder systems or any 100mm (4") holder. The filter is also available in 85mm, 150mm and 165mm sizes. Formatt Hitech filters are made from the highest quality 1.5mm optical resin.