Hard Edge ND grads feature a neutral gray effect at the top of the filter that sharply transitions to clear at the center of the filter. The main function of a graduated ND is to balance the luminosity of the sky to the terrain below. The hard edge grad primarily used in photos that feature a defined and uninterrupted horizon, such as a seascape. Formatt-Hitech filters are made from the highest quality 4mm Schott Superwite glass.