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.

Elliptical polarization

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.

Elliptically polarizedlight

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.

HOLBOURN, A. Angular Momentum of Circularly Polarised Light. Nature 137, 31 (1936). https://doi.org/10.1038/137031a0

Circular polarised lightmeaning

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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.

BY suspending a 6 mm. diameter circular half-wave plate horizontally from a quartz fibre with a torsional constant of 2 × 108 dyne cm. per radian, and allowing circularly polarised light to traverse it, thus reversing the direction of rotation of the electromagnetic light-vectors, we were able to observe torques of amounts plus and minus 2 × 1011 dyne cm., according to the direction of rotation of the incident light. The circular polarisation of the incident light was produced by passing plane polarised light through a quarter-wave plate. By rotating this quarter-wave plate through 360°, the light could be polarised twice in a clockwise direction and twice anti-clockwise.

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.