Long exposure

Long exposure means leaving the shutter open long enough that movement becomes a trace: water into a surface, cloud into streaks, passers-by into smears. In daylight that needs a grey filter – otherwise the frame is white before anything flows.

Where it begins

There is no fixed threshold. In practice long exposure starts where the time is longer than you can hold the camera – roughly a thirtieth of a second on a wide lens, a hundred and fiftieth on a long one. From there you need a tripod, and from the tripod on the way you work changes.

The look usually only arrives past half a second. Below that movement just goes soft; above it, movement becomes a shape.

What you need

• A tripod that stands in wind: The most common cause of a soft long exposure is not a focus error but a tripod that is too light. Leave the centre column down and hang weight on the hook if there is one.
• A grey filter in the right strength: An ND64 for seconds, an ND1000 for minutes. Which strength suits which subject is further down.
• A remote release or the self-timer: Pressing the shutter transfers into the frame. Two seconds of delay is usually enough; bulb mode needs a release that counts or a timer.
• A microfibre cloth: Spray settles on the filter at the coast, and one drop on the glass appears in every frame of the series. Take a look before each shot.

The order at the scene

• Frame and focus without the filter: Set up, choose the frame, focus. Then switch to manual focus so nothing drifts.
• Meter without the filter: Set aperture and ISO, note the shutter speed the camera suggests. That speed is the starting time for the calculator.
• Work out the time: Enter the starting time and the filter above. The result is the time you dial into the camera.
• Fit the filter, release: Only now screw the filter on – earlier it would disturb metering and autofocus. Past 30 seconds switch to bulb mode.
• Check the first frame: Read the histogram, not the preview. On the screen everything looks too dark in sunlight.

Which time for which subject

• Creek, small waterfall: 0.5 to 2 seconds: Water turns silky and keeps its structure. Expose longer and it becomes a white surface with no sense of direction.
• Sea, coastline: 10 to 30 seconds: Waves disappear and the surface goes smooth and misty. Under five seconds individual crests stay put, which usually looks restless.
• Drifting cloud: 1 to 4 minutes: Cloud only turns into streaks past roughly a minute. That takes ten stops and more, which means bulb mode.
• Making people disappear: 10 to 30 seconds: Anyone walking through becomes invisible; anyone standing still becomes a shadow. On a busy square it often takes several tries.
• Car light trails: 5 to 20 seconds: This usually works at dusk without a filter. In daylight there are no trails, because the headlights do not register.

Past 30 seconds: bulb mode

The longest built-in shutter speed on almost every camera is 30 seconds. Anything beyond runs in bulb mode: the shutter stays open while the release is held, or until it is pressed a second time. Somebody has to time it.

A remote release that counts will do it, and so will a timer on a phone. What matters is that the count does not stop while the shutter is open – at the other end is a frame you cannot repeat, because the cloud has moved on.

The most common mistakes

• Filter fitted before focusing: Past six stops autofocus finds nothing. Reverse the order and the focus holds.
• Viewfinder left open: On an SLR, light enters through the eyepiece and lifts the frame unevenly. That is what the supplied cover is for.
• Stabilisation left on: On a tripod it hunts for a movement that is not there and creates one doing so. Switch it off.
• White balance on auto: Every frame gets a different cast, and with strong filters the automatic is wrong anyway. A fixed value keeps the series consistent.

Long exposure is less a question of kit than of order: meter, calculate, fit the filter, release. Keep to that and all it takes is a grey filter, a tripod and a little patience.

Every value is in the ND filter table, the FAQ covers common questions, and Which ND filter? takes you from the shot to the number of stops.