I'm planning to photograph flowing river water with long exposures for a blurry effect. I currently have a 4-stop ND filter which may be good for shooting wide open in sunlight, but is probably not enough for photographing flowing water in sunlight. I have found that both 9-stop and 10-stop ND filters are available where I live (there are probably other options but not all of them are shipped to my country).

According to the sunny 16 rule, ISO 100 and f/16 requires 1/100 s exposure. If stopping down to f/22, 1/50 s exposure is needed. From that, one can see that 9-stop ND filter allows 10 s exposure whereas 10-stop ND filter allows 20 s exposure. If shooting at wider aperture like f/8, 9-stop filter would give 2.5 s exposure whereas 10-stop ND filter would allow 5 s exposure. The wider aperture would allow lower diffraction.

It appears to me that 10 stops would be better in direct sunlight, but is that so? Are there any benefits in a 9-stop ND filter such as ability to use autofocus?

I know that autofocus on this camera works barely in conditions where optimal parameters are ISO 6400, f/2.8 and 1/40 s exposure. Sunlight (ISO 100, f/16 and 1/100 s exposure) is 5200 times brighter than the low-light conditions where I have tested autofocus. Thus, I assume that autofocus should work in direct sunlight even given a 10-stop filter. However, lighting is not direct sunlight all the time. In overcast conditions, f/8 is needed instead of f/16, so light is one fourth of direct sunlight, or 1300 times brighter than the low-light conditions where I have tested autofocus. Thus, I assume autofocus should barely work even in overcast conditions with a 10-stop ND filter. However, the margin isn't huge, so I may run into AF problems.

Also, at this time of the year, my shortest exposure has been ISO-100, 1/500 s, f/2.8 (equivalent to ISO-100, 1/16 s, f/16, i.e. about one sixth of direct sunlight), but when the seasons change the sunlight will become more direct. Anyway, 10-stop ND filter in this light level is not guaranteed to work with AF as the light level is only 800 times the one where I tested autofocus to work.

My worries with 9-stop filter are that if shooting in sunlight, I might not get long enough exposure, and also it might be necessary to stop down to f/22, causing too much diffraction. However, the 9-stop filter could perhaps be combined with a 4-stop filter in demanding (very bright sunny) conditions, giving a 13-stop filter which is surely enough for anything except autofocus. Also, I can take multiple exposures with a 9-stop filter and combined them later.

My worries with 10-stop filter are that if not shooting in direct sunlight, autofocus might not work with a 10-stop filter but might work with a 9-stop filter. Of course, I can always focus without the filter and then put the filter on, but that might be inconvenient. Also, a 10-stop filter has a large gap when compared to 4-stop filter I already have, so if photographing near sunset time, the conditions may change to be such that 10 stops is too much but 4 stops is too little. Additionally, 10+4 or 14 stops is probably too much even in very bright sunny conditions.

So, which filter should I choose to complement a 4-stop ND filter? The 9-stop or the 10-stop one?

The minimum ISO of the crop sensor camera is 100, and the maximum exposure time is 30 seconds, although I can always take multiple exposures and average them during post-processing. I have the ability to use a remote shutter control for the camera, so one exposure doesn't automatically mean one shake of the tripod. If it matters, the lenses are Canon 24mm f/2.8 - f/22 and 50mm f/1.8 - f/22 and perhaps 85mm f/1.8 - f/22. So, there's at least 62x dynamic range in the aperture control; the difference between 4-stop and 9-stop would be 32x and the difference between 4-stop and 10-stop would be 64x.

Related, some general advice about ND filters: What are neutral density filters and how do I use them to create long exposures in daylight? ...where the author of the accepted answer is said to use 1000x or 10-stop filter.

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    Why are you concerned about AF? You’ll be using a tripod. Get focus, attach filter, shoot. – OnBreak. Apr 7 '19 at 8:07
  • Well, I guess it's easier to continuously have the filter on, but I guess the few seconds to take filter off and put in on back again aren't too slow compared to waiting for the exposure to end... Anyway, there's a non-zero probability I drop the filter to the river if I have to continuously remove it! – juhist Apr 7 '19 at 8:15
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    Why not use a variable ND? Then you could dial it back to get your focus, without having to remove it, & dial it in to take the shot. You could also then choose whether to use 9-stop or 10, or anything else... – Tetsujin Apr 7 '19 at 11:13
  • Do you have any other ND’s already? Are you using a square kit or circular? Do you think you’ll ever want to go longer? (I didn’t , until I did and started really liking ~10min exposures) – OnBreak. Apr 7 '19 at 15:18
  • Formatt-Hitech even has 16-stop and above (up to 25-stop, IIRC) filters in their Firecrest line... – twalberg Apr 7 '19 at 22:51

It doesn't matter much which filter you get. If you get the "wrong" one, you will have gained invaluable XP that will contribute to your continued advancement in photography.

Options to consider:

  • Don't get either filter. Since you will be using a tripod, you can use your current filter (or no filter) to take and blend multiple exposures. You can also play with other blending modes to decrease noise or increase resolution. (This is what I would do since I so infrequently dabble with long exposures.)

  • Get the 10-stop filter, as scottbb recommends. Given his apparent experience with the topic, it would be reasonable to defer to his judgment.

  • Get both 9-stop and 10-stop filters. You can delay your decision making until the moment before you take the photo.

  • If there is a significant price difference, get the one that costs less. However, there is the saying, "Buy cheap, buy twice." (Perhaps this is why I have way more lenses than I can reasonably use.)

  • Get a variable ND filter, as Tetsujin suggests. This would be the most versatile option, but now you have nearly infinite stops of light reduction to choose from. Is 9 seconds long enough? Or should you go with 10 seconds?

    My understanding is a properly constructed variable ND filter should limit cross-banding. But it seems like a potentially significant enough problem that it might be best to avoid them, as scottbb suggests.

  • Get an ND filter set, perhaps a square filter kit, as Hueco suggests.

  • Flip a coin. See TED: Barry Schwartz on the Paradox of Choice

If you need longer exposures:

  • Close the aperture a stop.
  • Decrease ISO.
  • Add your existing 4-stop ND filter.

If you need shorter exposures:

  • Open up the aperture.
  • Increase ISO.
  • Switch from the 9/10-stop ND filter to your existing 4-stop ND filter.
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TL;DR: Get the best 10-stop filter you are willing to afford. There are several to choose from, compared to shopping for a 9-stop filter.

In no particular order, some considerations:

  • This is very specific to the exact location you're shooting, but in my experience, the waterfall scene is darker than the Sunny 16 rule indicates. Usually, there's a bit of canopy surrounding the waterfall on either side (at least at the bottom of the falls, where you'll almost certainly be shooting from). Of course, if you're shooting in places like Iceland, with little/no tree cover and the falls are in otherwise open terrain, ignore this point.

  • While a smaller aperture will give you greater depth of field, and also allow you to slow down your shutter speed even more, it will also start to exhibit the diffraction limit of your system. (See: What is a "diffraction limit"?, other questions with the tag)

    As a rough guide, I only stop down to ƒ/22 when I'm looking to get beautiful diffraction spikes from the sun (either by itself at sunrise/sunset, or when it pokes out from behind leaves and other cover in midday). If I'm not going for the diffraction spikes, I try to go no slower/smaller than ƒ/11, maybe ƒ/16 depending on the lens.

    But because I can't stop down the aperture to slow down the shutter where I want it for long-exposure shots, that usually means I err towards darker / more stops on my ND filters.

  • Filter makers don't call it the "Big 9" or "9 Slow" or whatever. It's the 10-stop "Big Stopper" or 16-stop "Super Stopper", or Singh-Ray's 10- and 15-stop "Mor-Slo". Jus' sayin'. In general, when it comes to ND filters, in very rough terms, the more, the better.

    yes, yes, I know that Singh-Ray also has a 5-stop "Mor-Slo". But that doesn't help my point, so I'm ignoring it. =)

  • The main choice you should be considering is between 10- and 13- or 15/16-stops, not 9- vs. 10-stops. Generally, it's not hard — on the margin — to make up or give up a single stop: fudge 1/3–1/2 stop on the aperture, maybe drop the ISO 1/3–1/2 stop, et voilà. Or even in the moment, perhaps you really can't change the ISO (you're already at 100, you don't want to go lower), but just maybe some clouds might roll over, giving you those few seconds where the exposure drops a stop or two, so that when averaged over a 5–10 second exposure, you gain the stops you need.

    I've never been in a situation where I couldn't compensate for my ND filters being too dark (i.e., just up the ISO as a first reaction (although if shooting with Canon DSLRs, consider only shooting full-stop ISOs). But I've had plenty of situations where I wished I had more ND, even by just one or two stops.

  • Regarding variable ND filters: personally, ugh. Don't get me wrong, they have their place. And in my opinion, that place is at the front end of a 35 mm or narrower (full-frame equivalent field of view) lens shooting video.

    Understand that because of the frame rate (24, 30, or sometimes 60 fps), and especially shutter angle (roughly, duty cycle of the frame rate), shutter speed is largely removed from the videographer's toolkit when it comes to exposure. So the normal ISO + shutter speed + aperture trifecta of exposure control variables for photographers is reduced to just ISO + aperture in videography. But that's not enough. That's where ND filters, and especially variable ND filters come into play — it re-introduces another control variable, to allow for better choices of depth of field and/or ISO noise (with the caveat that variable NDs only allow for negative stops of control).

    Variable NDs seem like a great choice — after all, they provide a continuous range of light control — but only within their range. Most of them vary between 1.5–2.5 stops on the low end, to 8–10 stops at the upper end. But there's one aspect of variable ND filters that cannot be avoided with wide angles-of-view: the cross-polarization X-pattern (See also: Why are my results with a variable neutral density filter poor?, and How does a variable ND filter work?):

    What I'm saying is, vari-ND filters provide a quick- and continuously-controllable stop variable, but with some hefty compromises. If your scenario justifies those compromises, great. But for landscape and waterfall photography, where time (i.e., spur-of-the-moment setting changes, run-and-gun style) is most certainly not of the essence, those compromises are not justified. Slow down, take your time — after all, your shot is literally just sitting there, waiting for you to take several seconds (or tens of seconds) of exposure. A single 10-stop ND filter introduces only a single additional element into the optical path, meaning fewer opportunities for reflections (between glass that probably is not as good, or as reflection reducing, as the elements in a decent lens), than any vari-ND filter with two pieces of glass in front of your lens.

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A related question is whether you actually need 10 or 20 second exposures. Anything over a second or so will already look pretty smooth and you might find that you get more interesting results in the 1/4 or 1/8 range where there is still a hint of the original waves.

You'll probably find no practical difference between 9 and 10 stop filters. You can compensate either way with aperture most of the time.

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  • That's actually a very valid consideration: I can already achieve 1/4 s - 1/8 s with my existing filter, and if not in direct sunlight, perhaps 1 s could be achievable as well, albeit with lots of diffraction. I actually made the decision I start shooting with my existing filter (have to purchase two step-up converters, shame on you Canon for not standardizing the filter diameter size!), and purchase the 9 or 10 stop filter later only if I really need it. – juhist Apr 8 '19 at 11:03
  • @juhist honestly, if you’re going to start dumping money into filters, do check out square filter kits. Far more practical to use, especially when using a CPL and 10+ND. – OnBreak. Apr 8 '19 at 14:23

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