I have 3 ND (neutral density) with values of 2, 4 and 8.
I know these filters remove light levels, but I don't know when I have to use each filter.
Thanks for your time!
It all depends on the specific exposure you're trying to make. Generally when you're using an ND filter, it's because you're in a situation where the shutter speed and aperture setting combination you want to use will overexpose the shot.
The key is to know how much that combination overexposes the shot, in stops (EV). If you look at the histogram of an image you took, those bars on the histogram represent stops. If you want to shove your whole histogram to the left by two bars, then that means you overexposed by two stops, so, you could either use 1/4 the iso, 1/4 the shutter speed, or stop down your aperture 2 more stops (or any combination of the three settings to reduce by two stops), or slap that ND4 on the front of the lens.
ND filters are designated one of two ways--either by the number of stops it is, or by how much it reduces the light. Your filters, because they're given in powers-of-two are going by darkening factor. To translate to stops, just figure out the log2, so your ND2 is a 1-stop filter, the ND4 is a 2-stop filter, and the ND8 is a 3-stop filer.
If you were shooting in sunny-16 conditions (where good exposure would be iso 100, 1/100s, and f/16), but you want to use f/4 (which is four stops more than f/16), you'd stack your ND8 (three stops) together with your ND2 (one stop). With 1-, 2-, and 3-stop filters and stacking, you can choose from 1-6 stops.
Neutral Density filters are used to reduce the light so that you can have a larger aperture, or slower (longer) shutter speed, and still have a properly exposed photo.
ND2 equals a 1 stop reduction. ND4 equals a 2 stop reduction. ND8 equals a 3 stop reduction.
Just use the filters alone or even combined together to get the type of exposure you want.
One example would be using a a shutter speed of 10 seconds or longer to blur the movement of water in a waterfall or other body of water for creative effect.
ForestWander Nature Photography, www.ForestWander.com
The "trick", if you want to call it that, is: