An ND (Neutral Density) filter blocks some light and pass some light. Its color is gray, so it works evenly on all colors. However, way ND filters are labeled can be confusing. Allow me to explain:
In photography, we choose the f-stop as the increment of exposure change. We apply f-stop adjustments when we or camera automation changes the size of the entry opening of the camera lens. This is accomplished using a mechanical devise that expands or contracts the diameter of the entry pupil of the lens. This mechanism mimics the action of the human eye.
In the human eye, the pupil is surrounded by a pigmented (colored) portion called the iris after the Greek goddess of the rainbow. In the camera, we name this apparatus the iris diaphragm. Years ago it was decided that the increment of exposure was best served it doubled or halved the amount of light able to traverse the lens. Hens the f-stop adjustment is a 2X incremental change.
When closing down the iris of your camera you are likely to say, I just made a 1 or a 2 f-stop change.
As the number of f-stop changes progresses, the amount of light reduction becomes exceedingly grate.
Suppose you are photographing an object illumined by a 1000 watt lamp. Imposing a 1 stop filter cuts the effective lamp brilliance in half to 500 watts. A 2 stop ND cuts the 1000 watt lamp to 250 watts.
Take a look at this table that details the effect of a 1000 watt lamp:
1 stop = 500 watts
2 stops = 250 watts
3 stops = 125 watts
4 stops = 60 watts
5 stops = 30 watts
6 stops = 15 watts
7 stops = 8 watts
8 stops = 4 watts
9 stops = 2 watts
10 stops = 1 watt
So if you impose an 8 stop ND and the effective output of a 1000 watt lamp is only 4 watts. Impose a 10 stop ND and the 1000 watt lamp’s output is 1 watt.
With strong ND's like 8 or 10, the exploring light level is incredibly dim.