Kenko PRO ND filters are a high precision density filter series with a special multi-layer membrane using vacuum vapor deposition technology. It is used when you want to only adjust the amount of light without affecting the color of the photo. ND4 adjusts the amount of light by 1/4 (25% transparency) by 2 stops. The ND filter effect allows for less light entering and shooting at a lower aperture than when shooting without ND filters.