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What Do the Numbers On ND Filters Mean?

ND Filters 4

What do the numbers on ND filters mean?

Author: Alexis Hartman

We’ve talked about what ND filters are, but you may still be confused about all the numbers. The world of photography deals a lot in numbers, and when it comes to neutral density filters, these numbers are known as “stops”.  A “stop” refers to either cutting the amount of light in half or doubling it.

Since ND filters are used to reduce the amount of light in a scene, each stop is always halving the amount of light. The larger the stop number, the more light the filter blocks out.

Where this gets tricky is knowing the mathematical order of operations for the different stops. For example, a 1-stop ND will reduce the light by 50% and a 5-stop will cut the light in half five times in a row. If you have a 1-second exposure without a filter and then put on a 1-stop ND filter, you have effectively halved the amount of light coming into your camera. In order to balance out the exposure, you’ll have to increase your shutter speed by doubling it. Now, your 1-second exposure becomes 2 seconds.

The table below demonstrates the exposure time compensation needed when adding different strengths of ND filters.

Exposure Time
ND Filter in Stops
New Exposure Time
1 second
1
2 seconds
1 second
2
4 seconds
1 second
3
8 seconds
1 second
4
16 seconds
1 second
5
32 seconds
1 second
6
64 seconds
1 second
7
128 seconds
1 second
8
256 seconds
1 second
9
512 seconds
1 second
10
1,024 seconds (17 minutes)
1 second
15
32,768 seconds (9 hours)
1 second
20
1,048,576 seconds (291 hours or 12 days)

You can see how as the filter strength increases, the exposure time is doubled sequentially to compensate for the loss of light. When the light is cut in half, you need to double the shutter speed to maintain the same exposure. Add another ND stop, double the shutter speed again.

ND filters come in many different strengths. Depending on the manufacturer, these strengths may be notated in one of three ways: stops, optical density, or ND factor. Stops (sometimes referred to as exposure value) are fairly straightforward in that they tell you exactly how many stops your exposure will be adjusted by. Optical density is essentially 0.3 x the exposure value. This is the least common labeling system on the market. The ND factor is often listed as ND2, ND4, ND8, and so on. These numbers refer to the amount by which the light is reduced, ie. ND2 halves the light while ND4 reduces the light to one quarter. The chart below shows the translation between the three labeling systems.

ND filter in Stops
Optical Density
ND Factor
1
0.3
2
2
0.6
4
3
0.9
8
4
1.2
16
5
1.5
32
6
1.8
64
7
2.1
128
8
2.4
256
9
2.7
512
10
3
1024
15
4.5
32768
20
6
1048576

When searching for ND filters, we believe that labeling with the number of stops is the easiest way to understand exactly how they will affect your images. Our Kolari Pro ND Filters come in a wide range of strengths from 1-stop to 20-stops, and our Kolari Pro VND is available in 2-5 stops and 6-9 stops options.

Here’s a chart with everything put together:

Optical Density
ND.number
NDfactor
Lens area opening
f-stop Reduction
Fractional Transmittance
0.0
1
0
100%
0.3
ND 0.3
ND2
1/2
1
50%
0.6
ND 0.6
ND4
1/4
2
25%
0.9
ND 0.9
ND8
1/8
3
12.5%
1.2
ND 1.2
ND16
1/16
4
6.25%
1.5
ND 1.5
ND32
1/32
5
3.125%
1.8
ND 1.8
ND64
1/64
6
1.563%
2.0
ND 2.0
ND100
1/100
6 2/3
1%
2.1
ND 2.1
ND128
1/128
7
0.781%
2.4
ND 2.4
ND256
1/256
8
0.391%
2.6
ND400
1/400
8 2/3
0.25%
2.7
ND 2.7
ND512
1/512
9
0.195%
3.0
ND 3.0
ND1024 (also called ND1000)
1/1024
10
0.1%
3.3
ND 3.3
ND2048
1/2048
11
0.049%
3.6
ND 3.6
ND4096
1/4096
12
0.024%
3.8
ND 3.8
ND6310
1/6310
12 2/3
0.016%
3.9
ND 3.9
ND8192
1/8192
13
0.012%
4.0
ND 4.0
ND10000
1/10000
13 1/3
0.01%
5.0
ND 5.0
ND100000
1/100000
16 2/3
0.001%

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