Long exposure

Learning goal

You will be able to deliberately record the movement in a scene with an exposure lasting seconds, even minutes, and turn water into soft silk, clouds into long streaks, and a crowd into an invisible flow. You will explain that every moving subject needs its own motion time to reach the look you want, and find, with stop math, the filter density and shutter speed needed to reach that time through ND calculation. You will explain through each subject's contribution of light to the frame why walking people disappear in a long exposure and why still ones remain, and plan crowd removal either with a single long exposure or by combining many short frames. You will manage the two technical problems long exposure brings, the dark frame that removes hot pixels and light leaking in through the viewfinder, and if there are people in the scene, check how different skin tones are recorded in a long exposure.

The question

One Sunday morning you go with a friend from the photography club to the stone bridge of an old town. A fast stream flows under the bridge, and over the bridge passes a crowd heading to the Sunday market. In the frame you shoot at one five-hundredth of a second, the stream is a rough surface made of thousands of frozen drops and foam; the bridge, too, is packed shoulder to shoulder with people. In a photo your friend shows you on their phone, though, the same bridge is completely empty, and the water under it flows like misty, soft silk.

Your friend takes a tripod and a dark, almost black filter out of their bag. How does water look like silk, how does a crowd disappear? The same bridge, the same water, the same people; is the only difference how long the camera looks?

Context

In earlier modules you got to know how the shutter speed freezes or blurs motion, how an ND filter allows a long exposure in daytime, and how a tripod keeps the camera still (Lesson 13.3). This lesson turns these pieces into a planned technique: predicting in advance how a given movement will look over a given time, calculating the light to reach the needed time, and solving the technical problems a long exposure brings.

Long exposure shows something the eye can never see: a span of time gathered into a single frame. The eye sees each moment separately; long exposure stacks seconds or minutes on top of one another. That is why the result is both real and surreal. Water looking like silk is an aesthetic choice; the same stream can also be a strong photo with frozen drops. What matters is deciding which one you want and reaching it deliberately.

Simple explanation

How much time does each movement need?

Motion time is the approximate exposure time a moving subject needs to reach the desired look. Different movements need different times. In a fast-flowing stream, even a few tenths of a second give the water a slight flow line; a second or two turns the water's surface into soft, textured silk; thirty seconds and more melts the ripples and foam completely, leaving a misty, smooth surface. Clouds moving with the wind are much slower; for them to leave long streaks across the sky, a minute or two is often needed. Waves breaking on the shore depend both on the time and on the moment it starts; several frames shot with the same settings come out different from each other.

These times aren't exact values but starting points; the speed of the water, the strength of the wind and how close the subject is to the frame change the result. A close, fast movement leaves a pronounced trail in a short time, while a distant, slow movement needs much longer. That is why in long exposure, shooting test frames at several different times and comparing them is the most reliable way.

How do you reach the needed time?

ND calculation is finding the shutter speed with a filter from the shutter speed without a filter, using an ND filter's density in stops. Since each stop halves the light, each stop of the filter doubles the shutter time. A three-stop filter stretches the shutter time to eight times, a six-stop filter to sixty-four times, and a ten-stop filter to about a thousand times. If on a sunny morning the unfiltered reading shows one sixtieth of a second, about seventeen seconds are needed for the same exposure with a ten-stop filter; multiplying one thousand twenty-four by one sixtieth of a second gives this result.

The calculation is done first, because once a dark filter is on, almost nothing can be seen through the viewfinder or on the screen, and autofocus may not work. The practical order is this: put the camera on the tripod, set the framing and focus without the filter, lock the focus manually, take the unfiltered reading, attach the filter and enter the shutter speed you calculated. For times beyond thirty seconds you use the camera's bulb mode and a remote release. Closing down the aperture and setting ISO to its lowest also lengthens the time, but closing the aperture down too far reduces sharpness through diffraction.

How does a crowd disappear?

Crowd removal is making moving people invisible in the frame with a long exposure. A frame's brightness is the sum of the light falling on each point during the exposure. In a two-minute exposure, a person passing a point on the bridge stays there only a second or two; they change a very small part of that point's light, and what is left behind is either nothing or a faint ghost. The stones of the bridge, on the other hand, make a full contribution to the frame because they stay there for the whole two minutes. That is why in a long exposure moving people disappear and still ones remain: someone sitting on a bench or standing in one place shows clearly in the photo.

The same result can also be reached by shooting many short frames and combining them afterward. Combining methods that, across dozens of frames shot from the same place, on a tripod, at set intervals, choose for each point a value at which it is empty most of the time, clean the people out of the frame. This method doesn't need an ND filter and cleans up the ghosts left by moving people better; but because frames are combined, the honesty question you discussed in the compositing lesson applies here too.

What are the technical problems of long exposure?

A dark frame is a second frame the camera shoots right after a long exposure, for the same length of time with the shutter closed, finding the hot pixels that come from the sensor's own heat and subtracting them from the main frame. In a long exposure the sensor warms up and some pixels glow as colored dots; the dark frame cleans these dots up. Its price is time: after a two-minute exposure the camera waits two more minutes. In fast-changing light, this wait means missing the next frame.

A viewfinder light leak is light entering through the back of the viewfinder on cameras with a mirror reaching the sensor during a long exposure and leaving faint spots or streaks in the frame. In a short exposure the photographer's eye covers the viewfinder; in a long exposure, because the eye has left the viewfinder, light from behind can leak in. Using the small cover that closes the viewfinder, or a cloth, prevents this problem. On mirrorless cameras this problem doesn't occur because the viewfinder is electronic.

If there are people in the scene, long exposure needs a different kind of care. It is hard for someone to stand without moving for twenty seconds; even while breathing, their face blurs slightly. If you want to show a person sharp within a long exposure, you need to ask them to lean against something and stand calmly without holding their breath, or shorten the time. Some dark ND filters add a slight warm or cool color to the light; this color shift stands out most on skin. Dark skin can shift from brown to gray or red, light skin from pink to yellow or blue. In the filtered frame you need to set the white balance again by a neutral surface and check the faces by enlarging them.

Visual evidence

You set up the tripod in the meadow beside the bridge. You place the framing so that the bridge's arch and the stream sit in the middle, and lock the focus on the bridge's stones. The unfiltered reading shows one sixtieth of a second at ISO 100 and f/11. You attach the ten-stop filter and round the shutter speed to fifteen seconds. In the first frame the stream has turned into soft silk, and the foam has left white streaks; but on the bridge, half-transparent, faint figures can still be made out.

You add a second, three-stop filter in front of the ten-stop one; seventeen seconds rise to eight times, to about two minutes and sixteen seconds. You switch to bulb mode and open a two-minute exposure with the remote release; you don't forget to close the viewfinder cover. You wait two more minutes for the dark frame. In the new frame the bridge is almost empty; only an old man leaning on the railing and watching the stream stands sharp, because he hasn't moved from his place at all for two minutes. His dark skin has leaned toward red with the filter's slight warm shift; when you set the white balance again by the bridge's gray stone, his skin returns to its own brown. The stream is now a misty, smooth mirror.

Common mistakes

Summary

Long exposure gathers the movement of a span of time into a single frame; each movement, fast water, clouds or waves, needs its own motion time to reach the desired look, and these times are starting points confirmed with test frames. In ND calculation each stop of the filter doubles the shutter time; a ten-stop filter turns one sixtieth of a second into about seventeen seconds, and framing and focus are set before the filter goes on. Moving people disappear because they stay in one place for a small fraction of the exposure, while still ones remain; the dark frame cleans up hot pixels but doubles the time, the viewfinder light leak is prevented by closing the cover, and skin in the scene is protected against the filter's color shift with a white balance set again.

Mini assignment

In this assignment you will work with a tripod and, if possible, an ND filter in a place with flowing water or a moving crowd; if you don't have a filter, you can do the same tests at dusk.

1. Shoot the same flowing water at one hundredth of a second, half a second, two seconds and fifteen seconds; write the look of the water next to each frame.
2. Note your unfiltered reading and calculate the needed shutter speed with the stop value of the filter you use; confirm your calculation by shooting.
3. Shoot a moving crowd first with an exposure of a few seconds, then with one of a minute or two, and compare how the people disappear; mark the still people who remain in the frame.
4. If you have a camera with a mirror, shoot one long exposure with the viewfinder open and one with it closed; if there are people in the frame, set the white balance by a neutral surface and check the skin by enlarging it.

Check yourself

• Can you estimate about how much time a movement needs to reach the look you want?
• Can you calculate the needed shutter speed from an ND filter's stop value?
• Were you able to notice the color shift of skin in filtered frames and correct it with white balance?

Glossary

Mini quiz

1. When the unfiltered reading shows one sixtieth of a second, about how long is needed for the same exposure with a ten-stop ND filter?

• A) 1 second
• B) 5 seconds
• C) 17 seconds
• D) 2 minutes

2. Why do people crossing the bridge disappear in a two-minute exposure?

• A) Because each of them stays at a point for only a very small fraction of the exposure and contributes very little to that point's light
• B) The filter erases the people
• C) The dark frame removes the people
• D) People don't reflect light

3. What is the reason the camera waits for the same length of time after a long exposure?

• A) To charge the battery
• B) To cool the filter
• C) To find focus again
• D) To shoot a dark frame to clean up hot pixels

4. In a frame shot with a ten-stop ND filter, a dark-skinned person's skin has leaned toward red. Which is the most fitting correction?

• A) Removing the filter and shortening the shutter speed
• B) Setting the white balance again by a neutral surface in the scene and checking the face by enlarging it
• C) Setting saturation to zero
• D) Raising the ISO

Answers

1. C — Ten stops are about one thousand twenty-four times; one thousand twenty-four times one sixtieth of a second is about seventeen seconds.
2. A — A frame's brightness is the sum of the light falling during the exposure; a subject that stays only a short time leaves no trace.
3. D — The dark frame finds the dots that come from the sensor's heat and subtracts them from the main frame.
4. B — The filter's color shift is removed with white balance; the skin returns to its own color.

Summary card

• Each movement needs its own time; confirm with a test frame.
• Each stop of the filter doubles the shutter time; ten stops are about a thousand times.
• Set framing and focus before attaching the filter, lock focus manually.
• Moving people disappear, still ones remain.
• The dark frame doubles the time; close the viewfinder; protect skin in the filtered frame with white balance.

Bridge to the next lesson

The shoot at the bridge went on until evening. When it got dark, your friend took a small flashlight out of their pocket and, during a long exposure, drew a circle in the air in front of the bridge. In the frame, a bright ring of light hung in front of the bridge, the stream and the dark sky; the hand holding the flashlight couldn't be seen at all. In the next frame they wrote the first letter of their name with the flashlight. How do you write in the air with light? The next lesson tells about the trails moving light leaves in a long exposure and about light painting.