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Photography neutral density filter equivalents

ByGarf created updated

There are several different ways one can express how much attenuation a neutral density (ND) filter offers. Why? It is because they are really describing the same physical attenuation, using different units of measurement. Think of an ND filter as a device that reduces light. That reduction can be expressed in several equivalent ways:

  • “It transmits 10% of the light.”
  • “Its optical density is 1.0.”
  • “It has a filter factor of 10.”
  • “It’s a 3.32-stop filter.”

All of those describe basically the same type of ND filter. Anyway this is my attempt to rationalise that and bring it all together on a table of equivalents.

ND 1 Number
ND Number
ND Filter Number
Relative Transmission
Optical Density
F-Stop Reduction
Trans. %
Fractional Trans.
Scientific Trans.
none
none
none
1
0
0
100
1
1.0000 × 10⁰
ND 101
ND 0.3
ND2
0.5
0.3
1
50
0.5
5.0000 × 10⁻¹
ND 102
ND 0.6
ND4
0.25
0.6
2
25
0.25
2.5000 × 10⁻¹
ND 103
ND 0.9
ND8
0.125
0.9
3
12.5
0.125
1.2500 × 10⁻¹
ND 104
ND 1.2
ND16
0.0625
1.2
4
6.25
0.0625
6.2500 × 10⁻²
ND 105
ND 1.5
ND32
0.03125
1.5
5
3.125
0.03125
3.1250 × 10⁻²
ND 106
ND 1.8
ND64
0.015625
1.8
6
1.563
0.015625
1.5625 × 10⁻²
ND 2.0
ND100
0.01
2
6.667
1
0.01
1.0000 × 10⁻²
ND 107
ND 2.1
ND128
0.0078125
2.1
7
0.781
0.0078125
7.8125 × 10⁻³
ND 108
ND 2.4
ND256
0.00390625
2.4
8
0.391
0.00390625
3.9062 × 10⁻³
ND400
0.0025
2.6
8.667
0.25
0.0025
2.5000 × 10⁻³
ND 109
ND 2.7
ND512
0.001953125
2.7
9
0.195
0.001953125
1.9531 × 10⁻³
ND 110
ND 3.0
ND1024 (aka ND1000)
0.001
3
10
0.1
0.001
1.0000 × 10⁻³
ND 111
ND 3.3
ND2048
0.00048828125
3.3
11
0.049
0.00048828125
4.8828 × 10⁻⁴
ND 112
ND 3.6
ND4096
0.000244140625
3.6
12
0.024
0.000244140625
2.4414 × 10⁻⁴
ND 3.8
ND6310
0.00015849
3.8
12.667
0.016
0.00015849
1.5849 × 10⁻⁴
ND 113
ND 3.9
ND8192
0.000122070313
3.9
13
0.012
0.000122070313
1.2207 × 10⁻⁴
ND 4.0
ND10000
0.0001
4
13.333
0.01
0.0001
1.0000 × 10⁻⁴
ND 5.0
ND100000
1 × 10-5
5
16.667
0.001
1 × 10-5
1.0000 × 10⁻⁵

“Trans.” in the context of the above table refers to transmittance.

Truly neutral?

There is another source of confusion, that is the use of the word “neutral.” A truly neutral-density filter should ideally have the same attenuation at all wavelengths. Whilst modern ND filters are pretty good, hardly any truly give equal attenuation at all wavelengths. Often it is a case of “good enough is perfect”.

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