The Tiffen Double Fog Filter is a great special effect filter to have on hand to enhance your photography. This filter produces a more natural heavy fog effect where objects remain sharper and contrast is substantially reduced. The Double Fog filter comes in a variety of strengths, the lower number indicates a lighter grades which takes the edge off excess contrast and sharpness. The higher numbers indicate heavier grades which create unnatural effects, as in a fantasy sequence or a dreamlike aura of mystery.