Filter ND1000 GreenL 10 stop

The use of an ND filter allows the photographer to utilize a larger aperture that is at or below the diffraction limit, which varies depending on the size of the sensory medium (film or digital) and for many cameras, is between f/8 and f/11, with smaller sensory medium sizes needing larger sized apertures, and larger ones able to use smaller apertures. 
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 (blur of water movement, for example) and the aperture set as needed (small aperture for maximum sharpness or large aperture for narrow depth of field (subject in focus and background out of focus). Using a digital camera, the photographer can see the image right away, and can choose the best ND filter to use for the scene being captured by first knowing the best aperture to use for maximum sharpness desired. The shutter speed would be selected by finding the desired blur from subject movement. The camera would be set up for these in manual mode, and then the overall exposure then adjusted darker by adjusting either aperture or shutter speed, noting the number of stops needed to bring the exposure to that which is desired. That offset would then be the amount of stop needed in the ND filter to use for that scene.