Vivitar VNDX37 37mm 1-Piece Camera Lens Filter Sets
Vivitar Filters are manufactured from high-quality solid optical materials.  Each individual filter is heat-treated to avoid any rare movement or distortion.  This Variable Neutral Density filter reduces intensity of all wavelengths or colors of light equally, giving no changes in hue or color rendition.  The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture, exposure time and/or blur of subject in different situations and atmospheric conditions.  Instead of reducing the aperture to limit light, the photographer can add a ND filter to limit light, and can then set the shutter speed according to the particular motion desired and the aperture set as needed.