ND numbers, stops and density
Three naming systems describe the same glass, which is why filter shopping is confusing. They convert like this:
| Stops | ND number | Optical density | Light passed | Also called |
|---|---|---|---|---|
| 1 | ND2 | 0.3 | 1/2 | ND 0.3 |
| 2 | ND4 | 0.6 | 1/4 | ND 0.6 |
| 3 | ND8 | 0.9 | 1/8 | ND 0.9 |
| 4 | ND16 | 1.2 | 1/16 | ND 1.2 |
| 5 | ND32 | 1.5 | 1/32 | ND 1.5 |
| 6 | ND64 | 1.8 | 1/64 | Little Stopper |
| 8 | ND256 | 2.4 | 1/256 | ND 2.4 |
| 9 | ND500 | 2.7 | ≈1/512 | ND 2.7 |
| 10 | ND1000 | 3.0 | ≈1/1024 | Big Stopper |
| 15 | ND32000 | 4.5 | ≈1/32768 | Super Stopper |
The arithmetic is one line: new time = base time × 2^stops. Optical density is just stops × 0.3, and the ND number is 2 raised to the stops. Marketing rounds all three, which is why an "ND400" is sold as 9 stops when 400 is closer to 8.6.
Working with a ten stop
Order of operations
- Compose and meter with the filter off. Note the shutter speed.
- Focus, then switch to manual focus so nothing re-hunts.
- Fit the filter, cover the viewfinder if your camera has one, and dial in the calculated time.
- Shoot one frame and read the histogram. Adjust, then shoot the real one.
What goes wrong
- Light leaks. Thirty seconds is long enough for light entering an optical viewfinder to fog the frame. Cover it.
- Rated ≠ measured. A "10-stop" filter is often 9.5 or 10.5. Learn your filter's real offset once and add it in the extra-stops field above.
- Colour cast. Strong resin and cheap glass filters go blue or magenta. Shoot raw, set white balance from a grey card afterwards, or correct it with a LUT — you can test one in the LUT preview tool.
- Vignetting. Stacked filters plus a wide lens equals dark corners. One strong filter beats two.
Water goes silky around 1/2 second, loses all texture past 5 seconds, and turns to mist beyond 30. Clouds need 30 seconds and up to streak. Pick the effect, then pick the filter — not the other way around.
Common questions
How do I calculate a long exposure with an ND filter?
Double the metered shutter time once for every stop of filtration. Three stops means eight times longer, ten stops means about a thousand times longer. The calculator above does it in real values, including the awkward ones your camera cannot dial in.
Is ND1000 the same as a 10-stop?
Yes. The ND number is the light-reduction factor and 2¹⁰ is 1024, so ND1000 is the marketing round-down of a 10-stop filter. The same filter may be labelled ND 3.0 in density notation.
Can I stack a polariser with an ND?
You can, and it is often worth it for water and foliage. Budget about 1.5 stops for the polariser and expect uneven skies on lenses wider than about 24 mm, because polarisation varies across a wide field of view.
Do I need bulb mode?
Beyond 30 seconds, on most cameras, yes — plus a remote release or an intervalometer, since holding the shutter by hand will shake it. The readout above tells you when you cross that line.
Does an iPhone need ND filters?
For stills, rarely — a phone can already reach 1/8000 and small apertures are fixed. For long exposures in daylight it absolutely does, because the shutter cannot go slow enough on its own. For video, an ND is the only way to hold a 180° shutter outdoors; see the shutter angle calculator.
Related tools
Long exposures, on the phone in your pocket.
Lucid gives your iPhone real shutter control with a locked ISO, a live histogram, and zebra stripes that show clipping before you commit to the frame.