please help me with ND filters

[oopsyaar]

mine is a Canon S3 IS, and i am buyin a lens adapter to fix filters to my camera..
the filter size required is 58mm. i've already given n order for circular polarizer and uv filter. To shoot some waterfalls in my locality i am looking for an ND filter. My doubts are..
1. What do u mean by ND2, ND4 and ND8 filters.. wats the difference from 0.6, 0.9 values?
2. i heard that two polarizers can be effectively used as ND filters.. i want to kno to wat level of ND filter value can i reach with 2polarizers.. and mainly are they two circular or linear polarizers..?
please help me with this doubt.
thanks in advance..

[OsmosisStudios]

I dont know how many polarizers youd need to get a ND value

the ND# is the amount of light that is blocked. the 0.# values are the amount of a "stop" difference between filtered, and unfiltered. I have an ND-8, and quite like it. The only thing is, its more expensive.

[gdanmitchell]

The different "ND" values give you an idea of how much light will be diminished by the filter. IIRC, a ND2 is one stop, a ND4 is two stops, a ND8 is three stops and so forth.

You can use a CP as a quick and dirty ND filter. IIRC a CP will get you something on the order of about 2 stops. Singh-Ray makes a two-CP ND filter than adjusts the amount of light by way of rotating the CP filters relative to one another. If you rotate them so that their angles of polarization are out of phase you can get something like eight stops.

I understand that this setup works fine - at least hte Singh-Ray version - but that you it is rather thick and won't work without vignetting on wide angle lenses. If you tried to accomplish this with your own two CP filters you would like have the same problem only more so.

Dan

Originally Posted by oopsyaar

mine is a Canon S3 IS, and i am buyin a lens adapter to fix filters to my camera..
the filter size required is 58mm. i've already given n order for circular polarizer and uv filter. To shoot some waterfalls in my locality i am looking for an ND filter. My doubts are..
1. What do u mean by ND2, ND4 and ND8 filters.. wats the difference from 0.6, 0.9 values?
2. i heard that two polarizers can be effectively used as ND filters.. i want to kno to wat level of ND filter value can i reach with 2polarizers.. and mainly are they two circular or linear polarizers..?
please help me with this doubt.
thanks in advance..