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Neutral Density Filter Names and Numbers: how to Read ND and GND filters description?

When we purchase a ND (Neutral Density) or GND (Graduated Neutral Density) filter, we need to choose the filter intensity we want. However, we often come across abbreviations that are difficult to understand. With this article we will clarify and try to explain what the abbreviations of ND and GND filters mean. What ND1000 / ND 3.0 and ND 10-stop mean?

ND and GND filters terminology

The fundamental characteristic of each ND and GND filter is to reduce the light reaching the sensor of your camera. Depending on how much the light attenuation is, the filter will have a different intensity. The names and numbers shown on the filter, whatever it is, will indicate the density of the filter.

Although other filter manufacturers sometimes use a specific name or number, NiSi uses all three main existing nomenclatures to facilitate its photographers:

  • Reduction in Stop
  • Filter Factor
  • Optical Density

Technically the filter intensity can also be measured in Transmittance (%), but we will not discuss this nomenclature as it is used only when it is necessary to calculate the optical density mathematically.

Reduction in Stop (2 Stop, 3 Stop, etc.)

The stop reduction is the easiest way to understand the filter intensity as it is a direct measure of the light attenuation towards the sensor. If we mount a filter with reduction equal to 2 stops, to have the same exposure that we would have without a filter, we will have to compensate by 2 stops the exposure with the filter. For example, if without filter we shoot for 1 second and get the desired exposure, with a 2 stop filter we will shoot for 4 seconds (starting from 1″ we add 2 stops: 1″ -> 2″ -> 4″).

Filter Factor (ND8, ND64, etc.)

Filter Factor is a simple representation of the factor by which the ND or GND filter will reduce the light entering the lens. For example a filter that reduces light by 1 Stop will have a filtration factor of 2 (ND2). 1 Stop light reduction means only half the light will reach the sensor, so its Filter Factor will be 2. You can think it as a fraction: 1/2. ND4 it will be 1/4 (one fourth of the light will reach the sensor).

The confusion in the use of this nomenclature arises from the fact that the numbers indicated are confused with the reduction in Stop, but this is not the case. That’s why when you read ND16 it doesn’t mean that the reduction is 16 stops, but 4!

As the light reduction doubles for every further stop reduction, we can say that where “x” is the stop reduction, the filtering factor is equal to “2x“.

A filter with 6 Stop reduction will have a filter factor of 26=2x2x2x2x2x2=64 (ND64).

Optical Density (0.6, 0.9, etc)

Optical Density is one of the most common way to identify ND and GND filters, although the calculation is not the most immediate. Without going into the mathematical detail, it will be enough for us to know that the optical density is the logarithm with base 10 of the filter factor.

For example, the optical density of a filter with stop reduction of 3 and filter factor of 8 (ND8) will be:

Optical Density = log1023 = log10 8 = 0.90308998699

which for simplicity will be 0.9!

Keep in mind that 1 stop corresponds to an optical density of 0.3. Then multiplying the stops by 0.3, you’ll have the optical density of the filter. For example, 10 stops equals an optical density of 3.0 (10 * 0.3).

ND filters names and numbers

ND filters conversion table

Here a practical table that collects all the different names and numbers for NiSi filters. If you are looking for a way to convert exposure times with different intensities you can refer to this article and the conversion table also in PDF.

Conversion table
Stop Reduction Filter Factor Optical Density
1 ND2 0.3
2 ND4 0.6
3 ND8 0.9
4 ND16 1.2
5 ND32 1.5
6 ND64 1.8
7 ND128 2.1
8 ND256 2.4
9 ND512 2.7
10 ND1000 3.0
15 ND32000 4.5
20 ND1000k 6.0
Discover all ND and GND filters from NiSi UK.

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