The PROND100 has a 6 2/3-stop light reduction equal to a ND optical density of approx. 2.1.
Hoya gray filters are color-neutral filters. They have many uses. They allow you to extend the exposure time when stronger stops are not possible
The PROND100 has a 6 2/3-stop light reduction equal to a ND optical density of approx. 2.1.