Originally posted by bikehead90 0.3, 2x

Any ND filter will be absorbing / cutting down on the amount of light it lets through, so you will need to

*increase* your exposure with such a filter to get a "normal" looking image - either by making it longer, upping the ISO value, or opening up your lens more. Opening the lens means using

**smaller** numerical values of the f-stop. For f-stops, the numbers go in the opposite direction of the size of the aperture opening. The smallest values we usually use (16 or 22) correspond to very small openings in the lens to let the light through.

In filter nomenclature, the 0.3 is essentially the log (base 10) of the light attenuation, which corresponds to a factor of 2 - hence, also, 2x - which is one stop of light attenuation

(In log notation, the 0.3 is the power of 10 that has the equivalent value of 2: 10 to the power of 0.3 is (very nearly) equal to 2)

Adding logs is the same as multiplying the values they represent, so ND 0.6 is 0.3 plus 0.3 or 2 times 2, which on a filter might also be labeled 4x - which would be a total of 2 stops.

0.9 = 0.3+0.3+0.3 = 2 x 2 x 2 = factor of 8, or 3 stops

For filters with more attenuation, this scheme might go as big as 1.2.

Typically, though, at some point for higher attenuation, the decimal point gets omitted, and whole numbers are used. Thus a "1" filter has an attenuation of ten to the power of 1, which is a factor of 10 - somewhat more than the factor of 8 which is 3 stops.

A filter value of 3 means ten to the third power, which is 1000, which is very nearly equal to 1024, which is 2 to a power of 10, or 10 stops.

So, about as close as it would matter, a ND 3 filter has an attenuation of 10 stops.

An ND 4 is 10,000, or more than 13 stops, and an ND 5 is 100,000 or not quite 17 stops.

ND 4 and 5 are useful for taking pictures of the sun.

Note that ND values of 0.3 and 3 are thus VERY far apart - 1 stop versus 10 stops

(sometimes all that old math class stuff turns out to be useful!)