This 4-stop, 77mm QuartzLine Solid ND and Circular Polarizer Filter from PolarPro is a filter that combines a solid neutral density effect with a circular polarizer effect. When in use, this filter's 16x filter factor provides a 4-stop exposure reduction. Its 1.2 density creates a darkening of the entire image, allowing you to photograph with a wider aperture or slower shutter speed than normally required.