How to shoot long exposures with an ND filter

Silky water, streaked clouds, a plaza where the crowd dissolves into ghosts: every one of those is the shutter left open for seconds while the world moves through the frame. The long-exposure lesson teaches the physics: the shutter does not freeze time, it draws it. This capsule is the daylight workflow, because daylight is where the technique dies without help: in full sun the light forces the shutter to a fraction of a second no matter how far you stop down, and the motion never gets time to paint. A neutral-density filter is the fix: dark glass that cuts the light without touching its color, buying you seconds in the middle of the day. The discipline is that a dense ND makes the camera blind: through ten stops of glass the viewfinder goes black and autofocus hunts on nothing. So the sequence inverts: compose, focus, and meter FIRST with no filter mounted, lock everything to manual, then mount the filter and CONVERT the exposure by arithmetic (each stop of ND doubles the shutter time). A metered 1/60th behind a 10-stop filter becomes sixteen seconds: double it ten times, count on your fingers, nobody does this in their head at first. Past thirty seconds you are in bulb mode, which needs a remote release to hold the shutter open, and at that point stability is the whole craft: the frame is only as still as the tripod, and a sixteen-second exposure records every wobble a heavy finger on the shutter button puts into it.

Starting points

Filter choice: 3-stop ND for soft blur in dim light, 6-stop for waterfalls and rivers, 10-stop for daylight seascapes and vanishing crowds. Denser is not better: it is just slower; pick the one that lands your metered shutter in the 1–30 second range.
Sequence: Compose, focus, meter: all BEFORE the filter goes on. Then switch to manual focus and manual exposure, mount the filter, and never trust a reading taken through dense glass.
The conversion: Each ND stop doubles the metered shutter time. 1/60s through a 10-stop filter: double ten times → 16 seconds. Use an ND calculator or the table in the filter box until the doubling is reflex.
Mode: Manual up to 30 seconds: where the 2-second self-timer keeps your finger off the body at the press. Bulb beyond that, which needs a remote release: the self-timer can fire a bulb exposure but cannot hold it open.
Stability: A solid tripod on solid ground, image stabilization OFF on a locked mount, and on an SLR cover the viewfinder: stray light leaking in through the eyepiece can fog a long daylight exposure.
Base settings: Lowest native ISO and a mid aperture (f/8–f/11): the ND does the darkening, so the other two settings stay where the lens is sharpest and cleanest.

In the field

Find the motion first: water, cloud, crowd. A long exposure with nothing moving is just a slow photo of a still scene.
Compose and focus with the filter OFF, then lock focus to manual.
Meter, note the shutter, and convert it by doubling once per ND stop.
Mount the filter without nudging the focus ring; switch to manual or bulb.
Fire with a remote (or the self-timer on sub-30-second manual exposures) and keep hands off the tripod for the whole exposure.
Check the histogram after the first frame: the conversion arithmetic is honest, but the light may have moved while you were setting up.

When it fails

A color cast, magenta or brown, across the whole frame: Dense ND glass is rarely perfectly neutral, and some filters leak infrared that sensors render as a muddy magenta. Shoot RAW so the cast is correctable, set a custom white balance with the filter mounted, and if one filter always casts the same color, profile it once and reuse the correction.
A soft frame despite the tripod: Something moved the camera during those seconds: a finger on the shutter, image stabilization hunting on a locked mount, or wind on an extended center column. Fire remotely, switch IS off, keep the tripod low: long-exposure sharpness is stability discipline, not lens quality.
Water or sky rendered as blank, detail-free white: The highlights clipped: either the conversion overshot or the light brightened mid-exposure. Re-meter, shorten the exposure or add a stop of ND, and watch the histogram: the blur averages moving water into one broad, even sheet of tone, and where that sheet clips it clips as a single featureless mass, so check the first frame's highlight end before trusting the conversion (blown white holds nothing for recovery).

Finished the sequence? Get the honest read.

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