High-speed photography

Learning goal

You will be able to sharply freeze a moment that is over in the blink of an eye, the tiny fraction of a second in which a drop hits water and opens a crown, or a balloon bursts. You will explain that what freezes movement is usually not the shutter speed but the flash duration, and why a flash at low power fires for a shorter time. In a dark room you will take the ambient light out of play and leave the exposure to the flash alone, and find how short a light you need by calculating how far a falling drop travels in a short time. You will lock the focus on the right point before the event starts, use a sound trigger or a light-beam trigger to catch the moment, and set the trigger delay with test frames. While freezing a person's swinging hair or water splashing onto their face, you will strike the balance between flash power and skin, and keep safety in mind in a wet setup.

The question

From the kitchen tap, a drop falls into a bowl full of water every few seconds. The moment the drop touches the surface, the edges of the water rise like a toothed ring, a tower of water shoots up from the center, and a tiny ball appears at the tip of the tower; then it all scatters. The eye sees this shape only as a flicker. Your friend from the club lowers the shutter speed to the shortest value the camera allows; the result is dark, and the drop is a stretched line; when they try to press the shutter at the right moment, they catch either an empty water surface or rings that have already scattered.

Your friend turns off the kitchen light, sets a small flash to its lowest power and places it beside the bowl, and shoots with a shutter speed that leaves the room dark. After a few dozen attempts, the crown, the column and the ball on top look sharp as frozen glass. How is a water drop frozen in mid-air? How does a small flash achieve what the camera's shortest shutter speed couldn't?

Context

In the shutter speed lesson you chose short shutter speeds to freeze movement (Lesson 8.4). But a drop opening a crown is so fast that even the shortest mechanical shutter speed can't keep up; on top of that, in such a short exposure very little light enters the frame. In the high-speed sync lesson, too, you saw that in the mode where the flash produces a fast train of light along with the curtain, what freezes movement is now the shutter speed (Lesson 19.5). This lesson turns that around: it makes the flash's own brief burst the real tool for stopping movement.

High-speed photography has to solve two problems. It needs a light short enough to stop the movement, and that light needs to fire at exactly the right moment. Timing can't be solved with human reflexes; that is why an important part of the job is getting the camera and the flash to catch the moment for you.

Simple explanation

What freezes the movement?

Flash duration is the time during which the flash burst emits light. A speedlight or a studio flash gives this light within a very short time, and the light falling on the frame during this time records the subject's state at that moment. In a dark room, when the ambient light is almost zero, only the flash's light enters the frame for as long as the shutter stays open. So even if the shutter speed is several seconds, what decides the exposure and freezes the movement is the flash duration.

On most speedlights, power is changed by adjusting how long the burst lasts. At full power the flash fires for the longest time; this time can be on the same level as the camera's shortest shutter speed, or longer. As power decreases the burst shortens, and at low powers it gives a light much shorter than the shortest shutter speed. That is why in high-speed work the flash is set to one of the lowest powers, and the lost light is made up by bringing the flash closer to the subject or using more than one flash. On studio flashes the relationship between power and duration varies from model to model; you need to learn how each one behaves from the flash's own documentation.

A simple calculation shows how short a light you need. A drop falling from a height of half a meter, ignoring air resistance, hits the surface at about three meters per second. In one thousandth of a second this drop travels three millimeters and its edges stretch noticeably; in one twenty-thousandth of a second it advances a little more than a tenth of a millimeter and looks sharp. The faster the movement and the tighter the framing, the shorter the light needs to be.

How is the light set up?

The setup starts in the dark. The room is darkened, or the ambient light is kept so low that with the chosen shutter speed, aperture and ISO, a frame shot without flash comes out pitch black. This check matters: if the ambient light leaves a trace in the frame, a blurred ghost forms around the sharp image the flash froze.

The flash is switched to manual mode. The pre-flash an automatic flash sends (Lesson 19.3) spends light on metering, changes power from frame to frame, and if a person is being photographed, can make them blink before the main burst. Manual power gives the same duration and the same brightness in every frame. The aperture is set to a middle or slightly closed value so that the drop and the crown fit within the shallow depth of field; the ISO is kept low.

The direction of the light decides the shape. A flash placed behind the water lights a colored card or a translucent background in front of it, and the drop shows with glass-like edges in front of this bright background. A flash coming from the side, on the other hand, brings out the folds of the crown and the ripples on the water's surface.

How is the right moment caught?

Pre-focusing is locking the focus manually on the point where the movement will happen, before the event starts. A pencil or the tip of a spoon is held where the drop will fall, the focus is set on it, then autofocus is turned off. The event lasts a very short time and autofocus can't keep up with it.

The simplest way of timing is to keep the shutter open for a second or two in the dark and fire the flash by hand during that time; but with a repeating event like a drop, this is a gamble with a low hit rate. Triggers take over this job. A sound trigger is a device that fires the flash the moment it detects a sound; it is used with events that make a sound, such as a balloon bursting, a glass breaking or an object hitting the floor. A light-beam trigger is a device that fires the flash by detecting the beam of light cut by an object passing in front of it; it fires the moment a falling drop or a flying insect cuts the beam.

Trigger delay is the adjustable time added between the trigger detecting the event and the flash firing. The light-beam trigger detects the drop a few centimeters above the surface; the drop reaches the surface only a little later, and the crown opens even later. If you keep the delay short the drop is still in the air, and if you keep it long the crown has scattered. The right value is found with test frames: the delay is changed in small steps, a few frames are shot at each step, and you continue until the desired shape is caught. Drops from the tap falling on the same point at regular intervals make these tests repeatable.

What needs attention when photographing people?

High speed isn't only for drops. A dancer's swinging hair, flour thrown into the air or water splashing onto a face is frozen with the same principles. But when photographing people, you need to strike a balance between flash power and skin. To freeze movement the flash is kept at low power; low power, on the other hand, means less light. Because dark skin reflects less light than light skin, at the same low power the face can stay dark. Closing this gap by raising the flash's power lengthens the burst, and the hair blurs again. A better way is to bring the flash closer to the face, add a second flash at low power, or put a white reflector on the shadow side of the face; this way each flash stays short and the skin is also correctly exposed. On light skin, on the other hand, a close, strong flash can leave bright spots on the forehead and the bridge of the nose; moving the flash a little farther away or softening it is enough.

In a shoot where water is splashed onto the face, the model keeps their eyes closed and knows before each attempt what will happen. In a wet setup, flashes, cables and sockets must be where water can't reach; when splashing water touches an electrical device, the danger isn't only to the equipment.

Visual evidence

You turn off the kitchen light and open the tap just enough for drops to fall every few seconds. You put a blue card behind the bowl, and in front of the card a flash set to one of its lowest powers. The camera is on the tripod, in manual mode; the test frame shot without flash is pitch black. You hold the tip of a pencil where the drop falls, lock the focus on it and turn off autofocus.

You place the light-beam trigger just under the tap. In the first frames the drop is still in the air, hanging above the surface like a pearl; you lengthen the delay a little. A few steps later the crown opens, then the column rises from its middle. The small ball on top of the column is sharp as glass in front of the blue background; there is no stretching at all at its edges.

The next day your friend asks for a frame of a dark-skinned dancer swinging their hair. At the single flash's low power the hair froze but the face stayed dark. Raising the power blurs the hair again; instead, you add a second flash close to the face, also at low power. The strands of hair are sharp one by one in the air, the dancer's skin is in its warm brown, their eyes open.

Common mistakes

Summary

In high-speed photography what freezes movement is usually the flash duration: in a dark room only flash light enters the frame while the shutter is open, and on speedlights the burst shortens as power falls, which is why the flash is used at low power and close to the subject. Focus is locked on the right point before the event; the moment is caught with a sound trigger or a light-beam trigger, and the trigger delay is set with test frames. When photographing people, to light dark skin the flash is brought closer or a second low-power flash is added instead of raising the power; in a wet setup, electrical devices are kept away from water.

Mini assignment

In this assignment you will freeze a moment at high speed in a dark room, with a flash and a container full of water.

1. Calculate how far a drop falling from a height of half a meter, at about three meters per second, travels in one thousandth and in one twenty-thousandth of a second.
2. Darken the room and confirm that the test frame without flash comes out pitch black; switch the flash to manual mode and low power, and lock the focus in advance on the point where the drop falls.
3. If you have a trigger, catch the drop in the air, opening a crown and with the column rising by changing the delay in small steps; if you don't have a trigger, keep the shutter open and fire the flash by hand, and note the hit rate.
4. Try the same setup once at full power and once at low power and compare the sharpness. If you photograph a person, assess together whether their face is correctly exposed and whether the flash power froze the hair.

Check yourself

• Can you explain why in a dark room it is the flash, not the shutter speed, that freezes movement?
• Were you able to catch different stages of the event by changing the trigger delay?
• If you photographed a person, can you describe how you set up correct exposure for dark or light skin without raising the flash's power?

Glossary

Mini quiz

1. In a dark room with the shutter open, what freezes a drop opening a crown?

• A) The camera's shortest shutter speed
• B) A high ISO
• C) A closed-down aperture
• D) The flash's short burst

2. To freeze movement better with a speedlight, what is done with the flash's power?

• A) It is raised to full power
• B) It is set to low power and the flash is brought closer to the subject
• C) It is left in automatic mode
• D) It is set to high-speed sync

3. A blurred ghost appears around a sharp drop. What is the most likely cause?

• A) The ambient light leaving a trace in the frame
• B) The flash duration being too short
• C) The trigger delay being long
• D) The focus being locked in advance

4. A dark-skinned dancer's swinging hair is frozen but their face is dark. Which is the most suitable solution?

• A) Raising the flash power to full power
• B) Lengthening the shutter speed
• C) Adding a second low-power flash or a reflector close to the face
• D) Switching to automatic flash

Answers

1. D — With no ambient light, only the flash's brief light enters the frame; it is what freezes the movement.
2. B — On speedlights, low power means a shorter burst; the lost light is made up by bringing the flash closer.
3. A — The ambient light records the movement for as long as the curtain stays open and adds a blurred trace around the flash's sharp image.
4. C — Raising the power lengthens the burst; a close second flash or reflector gives both short light and correct skin.

Summary card

• In the dark, the flash duration freezes movement; a frame without flash must be pitch black.
• On a speedlight, low power = a short burst; bring the flash closer.
• Lock focus before the event; turn off autofocus.
• Catch the moment with a sound trigger or a light-beam trigger; set the delay by testing.
• When photographing people, don't raise the power, add light; keep water away from electricity.

Bridge to the next lesson

After finishing the drop sequence, your friend got caught up in the full moon visible through the window. They wanted to shoot a side silhouette of their own face and place the moon's bright surface inside that dark profile. They could do this in an editing program by putting the two photos on top of each other as layers; but they had heard there was a setting in their camera's menu that combines two frames during the shoot. How do two photos combine in one frame? The next lesson tells about the camera's multiple exposure mode and how combining during the shoot is planned.