Which ND filter?
There is no general answer, but there is a short one: it depends on how long you want the exposure to be, and that depends on what should happen in the picture. Below is the path from the idea to the number of stops, and underneath a comparison of every common strength.
If it has to be just one
ND64, six stops. Strong enough for visible motion blur and for video in sunlight, not so strong that every frame ends up in bulb mode. If you only ever want glassy water, take the ND1000 instead. If you mostly shoot video, ND8 and ND64 together will get you furthest.
hours : minutes : seconds : milliseconds
Working it out
Not "which is best" but "what should happen in the picture". In descending order of how often it comes up.
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You shoot video and want natural motion blur
The 180-degree rule asks for a shutter speed of double the frame rate, so 1/50 s at 25 fps. Under cloud an ND8 gets you there; in sunlight you need the ND64. With both in the bag you are covered for almost any light.
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Moving water should gain texture but stay water
A creek, a waterfall, surf from close up: half a second to two seconds. That is an ND64 in normal daylight, an ND8 in shade or at dusk.
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The sea should go glassy and the clouds should drift
That takes eight seconds and more, so ten stops. This is the classic ND1000, the filter most people buy one for in the first place.
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You want a long exposure but the sun is high
At midday by the sea an ND1000 lands at two or three seconds, too short for the effect. Then you need twelve stops. That is the special case, not the normal one.
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You shoot wide open in sunlight
A portrait at f/1.8 on a bright afternoon runs past the fastest shutter speed many cameras offer. This is not about movement at all, only about taking light away: three stops usually do it.
The strengths side by side
The same starting speed of 1/125 s, four filters. The last column is why it is not simply a case of stronger being better.
| Filter | Stops | Factor | 1/125 s becomes | Typically for |
|---|---|---|---|---|
| ND8 | 3 | 8× | 0.064 s | video under cloud, a little movement in water, wide apertures |
| ND64 | 6 | 64× | 0.512 s | video in sun, waterfalls, a first taste of long exposure |
| ND1000 | 10 | 1024× | 8.192 s | glassy sea, drifting clouds, making people disappear |
| ND4096 | 12 | 4096× | 32.768 s | long exposure in full sun, empty public squares |
What to look for when buying
- Diameter and step-up rings: Buy for your largest lens and put step-up rings on the smaller ones. A set of rings costs little, a second filter costs a lot. It does not work the other way round: a filter that is too small vignettes, or will not fit at all.
- Screw-in or square: Screw-in filters are more compact and seal better; square systems allow graduated filters and quick changes. For starting out with one or two strengths, screw-in is the simpler choice.
- Colour cast: Up to six stops it is hardly an issue, from ten it almost always is. This is where better glass genuinely pays off, while at the weak strengths it barely does. Shooting raw lets you correct a lot of it.
- Variable ND, carefully: An adjustable filter appears to replace all of them. In practice it often shows a dark cross in the frame at the top of its range, and its markings are approximate, so you cannot really calculate with it. Convenient for video, not for precise long exposure.
- Stacking instead of a second filter?: Two filters add their stops, but also their glass surfaces: more stray light, and dark corners on a wide angle. Usable as a stopgap, not as a plan.
If you are unsure, start with an ND64 and add above or below it later. It is the strength that least often ends up unused in a drawer.
Every value from ND2 to ND8192 is in the ND filter table, and the FAQ answers the common questions.