The Aurora Aperture 39mm PowerXND 2000 Variable Neutral Density 1.2 to 3.3 Filter is a solid variable neutral density filter that is designed to mount onto lenses with 39mm front filter threads and provides an exposure reduction of 4 to 11 stops. The 1.2 to 3.3 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 rear ring is visually demarcated with dots that represent different density values while the front ring includes knurling to further enable fast and precise positioning.