The SLR Magic 77mm Variable Neutral Density 0.4 to 1.8 Filter is a solid variable neutral density filter that is designed to mount onto lenses with 77mm front filter threads and provides an exposure reduction of 1.3 to 6 stops. The 0.4 to 1.8 density creates a darkening of the entire image, allowing you to photograph with a wider aperture or slower shutter speed than normally required. The degree of density is easily controlled by rotating the front filter ring, helping you to predetermine the additional exposure length required. By slowing your exposure time or increasing your aperture, you are able to control depth of field and convey movement more easily. For easier, repeatable control over density settings, the front ring is visually demarcated with numbers representing different density values and includes a rotation lever to further help enable fast and precise positioning of the front ring.