ND filters help reduce the intensity of light when using a slower shutter speed and a larger aperture. When trying to reduce the intensity of light, ND filters can help control the difference between the sky and the ground. It softens harsh shadows, giving images more depth.
By slowing down the shutter speed, ND filters will cause an object to blur, giving the illusion of motion.
If you use a larger aperture, the ND filters keep you from having to stop down the lens too much. If you are trying to separate the background from the subject the ND filters helps with increasing the depth of field.
The most commonly used types of ND filters are the ND 0.03, ND 0.06, and the ND 0.09, which stop the lens down one, two or three stops respectively.
Another type of ND filter is the graduated or grad filter. The grad filter does not cover the entire frame. There are three types of grad filters: the hard edge, the soft edge and the attenuator.
When using a hard-edge grad filter, the transition from the clear area to the ND area on the glass can be abrupt. The hard edge filter is used in conjunction with a longer lens.
A soft-edge filter causes the images between the clear and ND glass to look as if they are bleeding into one another. This is the most commonly used type of grad filter.
The attenuator filter gives the illusion that the frame is gradually darkening from the clear area to the ND area and is most often used with a wide-angle lens.
The main benefit of using an ND filter is the ability to control the percentage of light coming through the lens. The ND filter allows one to focus on the subject and not the background by using the filter effect to blur out the background.
Movement when using a ND filter is limited. The camera must remain stationary or the movement of the camera will betray the presence of the ND filter. Limit your panning and other camera movements.
When determining the look of the images in your frame, decide how much light you want to control. The ND filters give you a sharper look by equalizing the images in the frame and increases your depth of field. ND filters allow images such as snow and sand to be filmed without being overexposed.