This 77mm K-Series Variable Neutral Density and Circular Polarizer Filter from H&Y Filters is a filter that combines a solid variable neutral density element with a circular polarizer element. The variable neutral density element provides an exposure reduction of 1.5 to 10 stops. Its 0.45 to 3.0 density creates a darkening of the entire image, allowing you to photograph with a wider aperture or slower shutter speed than would normally be 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.

https://www.bhphotovideo.com/c/product/1429321-REG/h_y_filters_kvc77_k_series_77mm_mrc_variable.html/overview