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For example, when I use ND1000 filter, I know theoretically I have to increase shutter speed by 10 stops. Does this apply regardless of different stop increment settings (full stop vs half stop vs 1/3-stop), or do they say 10 stop in full stop increment?

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    What exactly do the instructions say that came with the ND Filter? – Alaska Man Nov 25 '19 at 18:42
  • @AlaskaMan No instructions came with the filter. FYI, I use NiSi EX Color ND1000 (screw-in) – elquimista Nov 26 '19 at 15:34
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It sounds like you are confusing "stops" with "steps". A photographic stop is a doubling of light exposure.

If you can adjust your exposure by 1/2- or 1/3- stop increments, then you would need 2 or 3 increments adjustment (respectively) to achieve a 1 stop change in exposure. So when using a 10-stop ND filter, you would need 20 steps if you were adjusting in 1/2-stop increments, or 30 steps if using 1/3-stop increments.

If you were shooting a scene, without ND filter, and your exposure settings dictated a 2-second shutter speed, then a 10-stop ND filter would necessitate increasing the exposure by a factor of 210 (i.e., 1024). Thus, you'd set the shutter speed to 2 * 1024 = 2048 = 34.133... min = 34:08.

Note: a ND1024 filter is precisely a 10-stop (i.e., 210 = 1024 multiplier) ND filter. A ND1000 is actually a multiple of 1000, not 1024. Thus the change from 2-second non-ND exposure would be 2 * 1000 = 33.33... min = 33:20. However, the ratio difference of 1024/1000 is only 2.4%. In terms of stops, the difference is log2(1.024) = 0.034 stops, a trivially small exposure difference that you won't be able to discern between two images, and way below the smallest precision of any commercially-available DSLR or MILC camera.


Regarding fractional-stop ND filters, they aren't common, but they do exist. I use mostly Lee filters. While I don't own fractional-stop filters, Lee makes ND 0.45 (1.5 stop) and ND 0.75 (2.5 stop) filters. If one were to use either of those filters, the exposure settings would need to be adjusted by 21.5 ≈ 2.83, or 22.5 ≈ 5.66, respectively.


Now, regarding fractional-stop exposure settings in camera: it doesn't matter if your camera is able to set exposure settings in fractional-stop increments. Whatever the "correct"/desired exposure settings a particular scene calls for without a ND filter in front of the lens, you need to increase the total exposure by the number of stops as called for when using your filter.

For instance, let's say a particular ISO 100, ƒ/8 shot is metered for 1/50" . A 10-stop ND filter would still require increasing the shutter duration from 1/50" to 1/50 * 1024 = 20.48 seconds. I'm not aware of any cameras or shutter remotes that allow fractional seconds beyond 2.5", so you can just pick 20 or 21 seconds. The difference is only 0.07 stops (i.e., less than 1/10 stop, still less than any camera can control for).

Related questions and links:

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    And, given that the whole stop sequence is a series of approximations, and that the 10-stop ND filter may not be exactly 10 stops, but maybe 9.7 or 10.2, you might need anywhere from 19-21 or 29-31 steps, respectively. Or maybe even a little less/more... – twalberg Nov 25 '19 at 18:53
  • @twalberg especially so at the higher end of the ND filter range (10, 13, 16, etc.). The variation in optical density of 1-stop to say 3-stop filters is a lot less than the potential variation of NDs like the Big Stoppers. – scottbb Nov 25 '19 at 18:59
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    @elquimista Correct reasoning, close-but-not-exact math. Assuming the ND1000 filter is accurately 10 stops, then 10 doublings = 2^10 = 1024. So 2 * 1024 = 34.133... = 34' 8" (34:08). But your reasoning is correct. However, the exposure difference between a factor of 1000 and a factor 1024 is only 2.4%, which comes out to only 0.03 stops, well below the smallest precision any commercial DSLR/MILC can reproduce. – scottbb Nov 25 '19 at 20:15
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    BTW, the different ND filter notations are also covered here in Are neutral density ratings in 1/3 stops or are they really in 0.3 stops? and How to read ND filter description? – Michael C Nov 25 '19 at 22:08
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    @MichaelC Yeah. I know you've made the argument in the past (strongly, I might add) that "see also" links should try to direct references to questions at PSE. Or at least, to PSE questions first. I'm editing my answer now to basically address all of these comments anyways, including your reference links. Thanks for picking up my lazy slack. =) – scottbb Nov 25 '19 at 22:30

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