ND (Neutral Density) filters have the characteristic of absorbing a certain amount of light and allowing you to lengthen the exposure times to the length you want (for example to make long exposures).
This feature makes ND filters unique, as it is impossible to recreate the exact same effect in post production. Even stacking shots, a technique to try to simulate long exposure, always presents the problem of artefacts and “ghosting”.
Another method to calculate or learn how to calculate shutter speed is a conversion table. To learn more about “ND filters” we also recommend our complete guide on choosing ND filters. If you simply want to consult the table you can skip the next paragraph, otherwise you can read below to find out how to calculate the exact shutter speed in your head or quickly with the calculator.
Calculate shutter speed
Calculating shutter speeds with an ND filter is a straightforward operation, requiring a few steps and a calculator (or strong mental arithmetic!):
- Take the shot to achieve the exposure you want and note the time. For example 1/60 of a second. If you are using a GND and Polariser as-well as dark “long exposure” ND. Take your test shot with everything except the dark ND in place.
- Take a filter, for example the NiSi ND1000, and note that it is called “ND1000”. “1000” indicates precisely how many times we have to multiply the shutter speed once the filter is mounted, as 1000 times less light will enter the lens. Consequently, we are going to lengthen the speed 1000 times to match our unfiltered exposure.
- If we have a fractional time, as in this case, we report the value metrically. So 1/60 of a second is 0.016″. A simple calculation, 1 divided by 60.
- Multiply this value by 1000, that is 0.016 * 1000 = 16.6″. The exact shutter speed to get a photo with the same exposure by mounting the ND1000 is about 16 seconds, in camera we can use 15 seconds which is the closest we can set. Or using a remote control you can shoot for exactly 16 seconds.
- If we only have an ND256 (8 stops) available? The calculation would be 0.016 * 256 = 4.26. That is, about 4 seconds.
These calculations are not always practical in the field, so a simple table can help us as we will see below.
ND filter conversion table
Below you will find the conversion table: in the first column on the left you will find the shutter speed without ND filter. This is the time you have set on the camera to expose the photo correctly with only the dark ND absent. If you use GND filters and the polariser, which we highly recommend, it is better to have them mounted so the only difference will be the ND filter (we recommend you also read the guide to long exposures).
Look in the left column for the exposure time you got before installing the ND and then scroll to the right to find the correspondence of the ND and time you prefer. You can choose the time you want and consequently which ND filter to use (for example if we want a photo with the movement of the clouds it is likely that we will also shoot up to 120 seconds, or about 3 seconds to obtain a movement of the water that is not too choppy , and so on). Or you can find how long to expose using a certain ND filter.
The table scrolls right to display all the columns.
|Exposure time without ND filter||1 Stop (0.3), ND2||2 Stop (0.6), ND4||3 Stop (0.9), ND8||4 Stop (1.2), ND16||5 Stop (1.5), ND32||6 Stop (1.8), ND64||7 Stop (2.1), ND128||8 Stop (2.4), ND256||9 Stop (2.7), ND512||10 Stop (3.0), ND1000||11 Stop (3.3), ND2000||15 Stop (4.5), ND32000||20 Stop (6.0), ND1000K|
Let’s take a practical example, by mounting the GND filter and the polariser you get a shutter speed of 1/125 of a second, with good exposure. Two possible scenarios:
- You want to shoot for 1 second to capture the waves in the foreground that crash on the rocks, too long a time would make them “disappear” while at 1/125 the action is too “frozen”. Look for 1/125 on the first right column of the table, find this time on row 7 and then reading to the right look for “1 second” and find it under the 7 Stop ND128 column.
- Now you know that to shoot with those times you have to use a 7 Stop ND128 filter but you only have a 3 and 10 stop filter and you want to know what time to set, look for the value 1/125 in the first column and then read the result corresponding to the ND8 3 Stop and ND1000 10 Stop, on the right. You find that with the 3 Stop you have to shoot for 1/15 of a second, while with the 10 stop ND1000 for 8 seconds.