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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?

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3 Answers

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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 way to quantify the density of the ND filter. It basically relates to some esoteric optical quality that engineers probably like to toss around while having coffee.

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!

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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. – Itai Dec 1 '12 at 15:21
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BTW, the esoteric property is optical density and you are right that is is simply easier to read the stop difference. – Itai Dec 1 '12 at 15:37
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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 Rogers Dec 1 '12 at 20:12
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@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. – Olin Lathrop Dec 2 '12 at 14:46
up vote 8 down vote

For the ND's that use decimals (i.e. .3 .6 .9), each .3 is one stop less light that reaches the sensor. So, a .9 means a 3 stop deduction in light to the sensor.

For the ND's that use a number (i.e. 8X), they operate under the power of 2 exponentially. So, an ND 16 is a 4 stop deduction in light (2 to the 4th power is 16).

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up vote 0 down vote

There are two common ways of quoting ND filter strengths, and one less common:

  • 2x, 4x, 8x, etc. Sometimes these are referred to as ND2, ND4, ND8, and so on. These refer to the amount by which the light is diminished. An ND2 filter halves the light, while an ND8 filter reduces it to one eighth.

  • 1 stop, 2 stops, 3 stops etc. Sometimes these are referred to as EV, for exposure value. These are probably the most convenient measurement because they tell you how many stops they'll adjust your exposure by.

  • 0.3, 0.6, 0.9 etc. These are basically just 0.3 x the number of stops of EV.

Each stop of exposure value refers to a halving of light, so:

  • 1 stop = ND2

  • 2 stops = ND4

  • 3 stops = ND8

  • 4 stops = ND16

And so on.

Stacking multiple ND filters adds for stops, and multiplies for strength values.

So, ND500 sounds pretty good, but it'd be the same as stacking an ND16 and an ND32 (16 x 32 = 512; manufacturers round it to 500).

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