How to read ND filter description?
When I search on Amazon for eg.
neutral density 67mm, I get long list of different filters. Some of them are marked like: ND2, ND4, ND8 etc. I'm guessing this means 2-, 4- or 8-stops filter, am I right?
But what about filters, that say 0.6 or 0.9? What does this mean?
Is there any other thing (apart of stops and diameter) that I should also pay attention to when choosing a filter?
The number associated with an ND filter is actually the denominator (bottom) of a fraction.
So an ND2 filter should be thought of as 1/2 the amount of light being allowed through the filter. For example, setting the lens at f/2.8, and using an ND2 filter would make that an f/4 situation for a total of 1 stop difference.
ND4 filter is allowing 1/4 the light (which is half of ND2) thus a 2 stop difference.
Continuing, ND8 is 1/8 and three stops and, although I've never seen them, an ND16 is half as much light as ND8 so would be four stops less light.
The decimal numbers you mention (0.6, 0.9) are another system of quantifying the density of the ND filter. These numbers are the log (base 10) of the factor by which the light is reduced. (This is sometimes called the absorbance). So for example a 1 stop filter reduces the amount of light by a factor of 2, and log(2) = 0.3 so a 1 stop ND filter is ND0.3 in this system. Similarly 2 stops is 0.6 and 3 stops is 0.9. The combined effect of multiple filters is obtained by adding up the numbers. For example a 1 stop, 2 stop and 3 stop filter combined (6 stops in total) would be 0.3 + 0.6 + 0.9 = log(2^6) = log(64) = log(2) + log(4) + log(8) = ND1.8.
I would highly suggest the best quality GLASS filters you can afford. Cheaper (especially plastic) filters will tend to add nasty color effects. Although technically color casts can be corrected in post, cheap filters also can also reduce the quality of light meaning things like more chromatic aberation.
Lastly, don't worry about getting the highest ND number, I carry two filters around and stack them together, when needed, for combined affect. Which is more reason why quality filters matter as stacking simply magnifies imperfections too!
Color can be corrected but you wouldnt want to. Those things have bizarre shift across the frame, so its not like a global adjustment would be of any use.
BTW, the esoteric property is *optical density* and you are right that is is simply easier to read the stop difference.
Is it? It's logarithmic (and equivalent to the Bell). Shift the decimal point one to the right and it's decibels. Every .1 density is a third of a stop, or one click of either the aperture or shutter speed dial. (0.3 (or 3dB) is a full stop.) When you stack filters you only have to add the values (rather than multiply, as one does with filter factors). But you kids don't use colour filters, do you? Trust me, if you use external meters and shoot film, density values are easier in the field.
@Stan: Yeah, logarithmic makes more sense, but it's always bugged my that "density" is expressed as a logarithm of 10, whereas everything else in photography is expressed as a logarithm of 2, like f-stops. It seems Log10 density is used in the lab for measuring film, sensors, attenuators, and the like. But in the field when taking pictures we use Log2 (f-stops). I don't understand why filters aren't rated more relevant to their end use, which would be in f-stops of attenuation. When adjusting a camera, "3 f-stops" is more immediately useful than a factor of 8 or a density of 0.9.