LESSON 54
Long exposure and ND filters
Smooth water, streaking clouds, empty streets.
IN THIS LESSON
- What a long exposure does
- What an ND filter is
- Working out the time
- In practice: getting it clean
What a long exposure does
A long exposure keeps the shutter open for a second, a minute or longer, so anything that moves is recorded as a blur or a trail while still things stay sharp. Moving water turns to mist, clouds stretch into soft streaks, car lights become lines and walking people fade out of a busy square. At night long exposures happen naturally because there is little light. By day there is too much: even at the lowest ISO and smallest aperture, a ten-second exposure would be far too bright. That is the problem a neutral density filter solves.
What an ND filter is
A neutral density filter cuts the amount of light of all colours equally, so it darkens the image without changing its hue. Its strength is given as optical density. The fraction of light that gets through is ten to the power of minus the density, so a density of 0.3 passes about half, which is one stop. Each further 0.3 halves it again. Some filters reflect light with thin coatings; others absorb it in the glass. ND filters also let you use a wide aperture in bright sun, for shallow depth of field, when the camera's fastest shutter is not fast enough. The table is computed from the formula.
Working out the time
With a filter on, take the metered time without it and double it once for every stop the filter removes. A scene that meters at 1/60 second needs about 1/8 second behind a three-stop filter and about 15 seconds behind a ten-stop filter. Strong filters are so dark that you cannot see through the viewfinder or focus with them in place, so compose and focus first, switch to manual focus, then fit the filter. For times past 30 seconds you need the camera's Bulb setting and a remote or a built-in timer.
In practice: getting it clean
This section is working practice. Use a solid tripod and a remote release, turn off stabilisation, and hang your bag from the centre column in wind. Cover the viewfinder eyepiece on a DSLR during the exposure, because light leaking in from behind can fog the picture. Strong filters often add a colour cast; shoot RAW and set white balance afterwards. Check the histogram, not the screen alone. Watch for small things that move when you do not want them to, such as leaves, boats at anchor and people who stop for a moment and leave a ghost.
| Optical density | Light passed | Stops (approx.) |
|---|---|---|
| 0.3 | 1/2 | 1 |
| 0.6 | about 1/4 | 2 |
| 0.9 | about 1/8 | 3 |
| 1.8 | about 1/63 | 6 |
| 3.0 | 1/1000 | 10 |
Try it today
- 01Find moving water or clouds on a bright day.
- 02Meter and shoot without a filter at your lowest ISO.
- 03Fit an ND filter, recalculate the time, and shoot again from the tripod.
- 04Try three lengths: one second, ten seconds and thirty seconds.
- 05Compare how the water or sky changes, and pick the length that suits the scene.