
A neutral density (ND) filter reduces the amount of light that reaches the lens. An ND1000 darkens the image by about 10 stops, or about 0.002% of the light. The equivalent darkness would be achieved with, for example, two ND64 filters.
Usually, there is a shortage of light in photography, but sometimes there can be too much. In bright sunlight, using a large aperture can be impossible without an ND filter. A neutral density filter is also needed when you want to photograph, for example, flowing water in bright weather and with a really long exposure time.
The multi-coated Pro Series ND filter series has the same color balance regardless of the filter's darkness, so you can keep your white balance setting the same even when you change filters.
The filter end has a female thread that matches the mounting thread.