Nikonian72 Loc: Chico CA

Neutral Density Filters for Time Exposures:

In photography, ND filters are quantified by their equivalency to f-stop reduction, or light transmission percentage.

ND2 = 1-stop difference = 50% transmission (reduces light to 1/2)
ND4 = 2-stop difference = 25% transmission (reduces light to 1/4)
ND8 = 3-stop difference = 12.5% transmission (reduces light to 1/8)
ND16 = 4-stop difference = 6.25% transmission (reduces light to 1/16)

Waterfalls (as an example) need at minimum, 1/2-second exposure to show water movement. 1-second or longer, shows more silkiness. At lowest ISO setting (i.e. 100) at minimal aperture (i.e. f/22), the amount of sun on scene will determine your exposure, which may be too quick for silky water effect.

Using an ND2 will double normal exposure time.
Using an ND4 will quadruple normal exposure time.

Variable ND filters are actually two, stacked standard (not circular) polarizing filters. As you rotate one, the view will grow darker until you hit 90-degrees relative to one another. Continued rotation, the view will grow lighter until another 90-degrees is reached, when the polarizing lines are parallel. Quality VND filters are quite expensive, and have markings denoting 1/3 ND stops (see example below).

Here are professionally written articles:
Long Exposures and ND Filters
http://www.manfrottoschoolofxcellence.com/2011/03/timo-frey-long-exposures-and-nd-filters/#.VGbUIslNenI

Graduated Neutral Density Filters
http://www.manfrottoschoolofxcellence.com/2014/07/using-neutral-density-nd-filters/#.VGbUjMlNenI

Typical Variable ND filter


Soft Graduated ND filters