The Tiffen Attenuator / Blender screw-in ND filters have been designed to offer a fine degree of neutral light control attenuation on wide-angle lenses.
The smooth transition across the surface of the filter is designed to aid light control within your image area for a number of purposes, including maintaining a constant exposure range across your image when you have a bright sky.
This ND grad features a density strength of 0.
6 which allows you to increase exposure by 2 stops to suit individual situations.