Timelapse

Learning goal

You will be able to turn a slow change lasting hours into a video of a few seconds by lining up photos you shoot at set intervals one after another. Starting from the video length and frame rate you want, you will calculate how many frames are needed, how long the shoot will take with the interval you choose, and how many times faster the event will run. You will choose an interval according to the subject's speed, and set the shutter speed according to whether the movement should look smooth or choppy between frames. When the frames' brightness fluctuates you will find where the flicker comes from, change the exposure in small steps in a shoot where the light keeps falling, such as a sunset, and plan the shoot in terms of battery, card and shutter. If there are people in the frame, you will take into account that they turn into flickering shadows, that skin color shifts in changing light, and that recognizable faces raise a privacy question.

The question

Over the valley, clouds flow from west to east. Their shadows come down the slopes of the hills, darken and brighten the town's roof tiles by turns, then cross to the far bank of the stream. Looking with the eye, this movement can barely be made out; you don't notice a cloud changing place without watching it carefully for a few minutes. When your friend from the club shoots a single frame, the clouds are frozen, and in a long exposure the sky has turned into a soft, textureless cover.

Your friend puts the camera on the tripod and shoots a frame every four seconds for an hour. When they line up these nine hundred frames one after another at home, a half-minute video comes out: the clouds flow over the hills like a river, the shadows run across the valley. How do you show the movement of clouds? How does an hour of sky fit into half a minute?

Context

In the remote release lesson we defined interval shooting as the camera shooting frames by itself at set intervals (Lesson 13.4). In that lesson we only mentioned that this setting opens a door to time-lapse; this lesson goes in through that door. In the long exposure lesson, too, you showed movement as a trail in a single frame (Lesson 22.1). Timelapse shows the same movement another way: it doesn't gather the trail into a single frame, it makes the movement itself visible by compressing time.

Timelapse stands between photography and video. Each frame is a photo; the rules of exposure, white balance and composition apply. But the result is a video, and the frames are seen not one by one but dozens per second, one after another. That is why small inconsistencies not noticed in a single frame, a jump in brightness or a slight shift, turn into flicker or shaking that stands out in the video.

Simple explanation

How many frames, at what interval?

Timelapse is the technique of recording an event with frames shot at set intervals and playing these frames at normal video speed to show it in compressed time. The calculation has two ends: how long the video will be and how long the shoot will take. The two numbers that connect them are the interval and the frame rate.

Frame rate is the number of frames shown in one second of video; common values are twenty-four, twenty-five or thirty frames per second. The interval is the time that passes between two frames. The calculation is simple: a ten-second video at twenty-five frames per second needs two hundred fifty frames. If you shoot a frame every five seconds, these two hundred fifty frames take one thousand two hundred fifty seconds, that is, about twenty-one minutes. In one second of video, twenty-five frames, that is, one hundred twenty-five seconds of real time, pass; the event runs one hundred twenty-five times faster.

The subject's speed decides the interval. For clouds flowing fast in the wind, intervals of a second or two give smooth movement; for slowly drifting clouds or sliding shadows, five to ten seconds suit better. Stars turning in the sky or a flower opening need much longer intervals. These values are starting points; if the interval is too short the movement drags in the video, and if it is too long the clouds jump from frame to frame. If you aren't sure, shooting with a short interval is safer, because you can speed up the video later by skipping extra frames, but you can't put missing frames back.

Why does movement look smooth or choppy?

Each frame's shutter speed decides how much of a trail movement leaves in that frame. A very short shutter speed freezes each cloud sharp; when the frames are played one after another, the clouds advance in small jumps instead of flowing softly. A longer shutter speed slightly blurs the movement in each frame, and this blur links the frames to each other and softens the flow.

As a starting point, some photographers choose a shutter speed that covers about half the interval: a two-second exposure at a four-second interval, for example. In daytime, for a shutter speed this long you need to reduce the light; the ND filter you saw in the long exposure lesson is useful here too. This isn't a requirement: sharp, choppy movement is sometimes exactly the effect you want, especially when you want to show the bustle of people in a crowded square as energetic.

Where does flicker come from?

Flicker is the frames' brightness differing from each other by small amounts in a timelapse video, and the image fluctuating as if blinking when played. Looking at a single frame this difference isn't visible; when twenty-five frames per second pass one after another, the eye catches it at once.

The most common source of flicker is automatic exposure. The camera meters each frame again, and reacting to a cloud passing or a car's headlights, changes the exposure in stepped amounts. That is why timelapse is usually shot with manual exposure; white balance is also fixed, and focus set manually. The aperture blades can also produce flicker: as the lens opens and closes down for each frame, the blades may not stop at exactly the same point each time, and this small difference shows up in brightness. Leaving the aperture wide open, or using a lens whose aperture is set by hand, removes this source. There are also deflicker programs that smooth out the small fluctuations that remain.

The electronic shutter used for silent shooting prevents mechanical wear, but under fluorescent or LED light it can create bands in the frames and brightness that changes from frame to frame. The mechanical shutter, on the other hand, works for every frame, and long timelapse sequences use it up quickly; which one to choose depends on the light in the scene.

How do you shoot a sunset?

A day-to-night transition is a timelapse shoot in which the light keeps falling from sunset to darkness, requiring the exposure to be changed during the shoot. Fixed exposure doesn't work here: an exposure set for the sun stays pitch black in the dark, and one set for the dark blows out to pure white at sunset. Automatic exposure, on the other hand, brings flicker back.

The solution is changing the exposure by hand in small, infrequent steps. As the light falls, you raise the shutter time or the ISO every few minutes in small steps, such as a third of a stop. Each step leaves a slight jump in the video; these jumps are smoothed later with programs that bring the frames' brightness closer together. White balance also needs a decision: if you leave it fixed, the orange of the sunset and the blue of the night are preserved in the video; if you change it from frame to frame, the transition flattens. Which one to choose depends on what you want to tell.

If there are people in the frame, their skin changes as the light changes. With fixed white balance a face shifts to orange at sunset and to blue in the dark; this tells the story of the evening's light, but it can be misleading if it is used to introduce a person. As the light falls, a dark-skinned face turns into a silhouette earlier than a light-skinned face; raising the exposure while also checking the faces means not making the sky the only measure. In a crowd shot at long intervals, people turn into flickering shadows that appear and disappear from frame to frame, and faces are usually not recognizable. In a square shot at short intervals and up close, on the other hand, faces can be made out; before publishing you need to think about whether the owners of these faces know.

Visual evidence

You set up the tripod at the end of the bridge and hang your bag from its center to add weight. You place the framing so that two thirds of it is sky over the valley, and fix the focus and white balance manually. You switch the exposure to manual mode and leave the aperture wide open. The clouds are flowing at medium speed; you choose a four-second interval and lengthen the shutter speed to two seconds with an ND filter.

For a half-minute video at twenty-five frames per second, seven hundred fifty frames are needed; at a four-second interval this takes fifty minutes. In the test sequence the shutter speed was too short and the clouds advanced in small jumps in the video; with a two-second exposure the flow softens. As the sun goes down behind the hills, every few minutes you lengthen the shutter speed by a third of a stop. In the video the sky passes softly from orange to navy, and the town's lights come on one by one. The people walking on the bridge are transparent shadows appearing from frame to frame; no one's face can be made out.

Common mistakes

Summary

Timelapse compresses time by playing frames shot at set intervals at normal video speed; the video length and frame rate decide the number of frames, and the interval decides the length of the shoot and the speed-up factor; the interval is chosen by the subject's speed, and the shutter speed sets whether movement looks smooth or choppy. To prevent flicker, exposure, white balance and focus are fixed, and the aperture blades are kept from moving; in a day-to-night transition the exposure is raised by hand in small steps and the remaining jumps are smoothed. If there are people in the frame, it is also considered that skin changes color and brightness in changing light, and when faces are recognizable.

Mini assignment

In this assignment you will shoot a timelapse on a tripod, of a slowly changing scene such as the sky or shadows.

1. For a ten-second video at twenty-five frames per second, calculate how many frames are needed, how long the shoot will take with the interval you choose, and how many times faster the event will run.
2. Set the exposure, white balance and focus by hand and shoot the sequence; combine the frames in a video program at the frame rate you chose.
3. Shoot the same scene in short sequences, once with a short and once with a long shutter speed, and compare the smoothness of the movement.
4. If there are people in the frame, note whether faces are recognizable in the video and how skin looks as the light changes.

Check yourself

• Can you calculate how many frames are needed from the video length and frame rate, and the shoot length from the interval?
• If there was flicker in your video, were you able to find its source?
• Did you check whether the faces of the people in the frame were recognizable and how their skin looked?

Glossary

Mini quiz

1. If a ten-second video at twenty-five frames per second is shot with a frame every five seconds, about how long does the shoot take?

• A) Two minutes
• B) Five minutes
• C) Twenty-one minutes
• D) Two hours

2. In a timelapse video the image fluctuates as if blinking. What is the most likely cause?

• A) Automatic exposure metering each frame again
• B) The interval being short
• C) The frame rate being high
• D) The tripod not being level

3. In the video the clouds advance in small jumps instead of flowing softly. Which change softens the flow?

• A) Raising the ISO
• B) Lowering the frame rate
• C) Closing down the aperture and shortening the shutter speed
• D) Lengthening the shutter speed with an ND filter

4. In a timelapse from sunset to darkness, the face of a dark-skinned person on the bridge turns into a silhouette early. Which is the most suitable approach?

• A) Fixing the exposure for the sunset
• B) While raising the exposure in small steps, looking not only at the sky but also at the faces
• C) Switching to automatic exposure
• D) Lengthening the interval

Answers

1. C — Two hundred fifty frames times five seconds makes one thousand two hundred fifty seconds, about twenty-one minutes.
2. A — When the camera meters each frame again, brightness shifts in small steps; when played, this shows as flicker.
3. D — A long shutter speed slightly blurs the movement in each frame and links the frames to each other.
4. B — As the light falls, dark skin darkens earlier; the exposure steps need to be taken with an eye on the faces too.

Summary card

• Number of frames = video length × frame rate; shoot length = number of frames × interval.
• Choose the interval by the subject's speed; if you aren't sure, keep it short.
• A long shutter speed gives smooth movement, a short one choppy movement.
• Fix the exposure, white balance and focus; don't let the aperture move.
• At sunset, raise the exposure by hand in small steps; check faces and privacy too.

Bridge to the next lesson

When they got home in the evening, your friend noticed the kitchen tap dripping. Each time a drop fell into the bowl of water below, for an instant a small crown opened, a thin column rose from its middle, then everything scattered. The eye saw this shape only for the length of a blink. When they shot it with the camera's shortest shutter speed, the frame came out both dark and blurred; pressing the shutter at the right moment, on the other hand, was almost impossible. How is a water drop frozen in mid-air? The next lesson tells about high-speed photography, which freezes movement with a very short burst of flash and the right timing.