High-speed cinematography

Slow motion is one of the signature looks of commercials and music videos. A splash of juice that unfolds like a flower, hair that floats through the air, a sprinter's muscles rippling, a champagne cork leaving the bottle: these images are possible because high-speed cameras record many more frames per second than the playback rate. CINE-320 introduced the basic slowdown factor for documentary slow motion. This lesson goes much further, into the specialist world of cameras that shoot hundreds or thousands of frames per second. It covers how high-speed recording works, the light it demands, flicker, triggering and timing, data and workflow, and the crew who run these systems.

Frame rate, playback rate and the slowdown factor

When footage is recorded at a high frame rate and played back at a normal rate, time stretches. The slowdown factor is the ratio of the capture rate to the playback rate.

A shot captured at 1,000 frames per second and played at 25 frames per second is slowed by a factor of 40; one real second fills 40 seconds of screen time. At 24 frames per second playback, the factor is about 41.7.

The slowdown factor also tells you how long a real event will last on screen.

Kinds of high-speed cameras

Cameras that shoot high frame rates fall into broad groups. Cinematographers should understand the categories rather than brand names, because models change quickly.

• Cinema cameras with high-speed modes record 100 to 240 fps or more, often at reduced resolution or with a cropped sensor. They are convenient because the look matches the main camera.
• Mirrorless and compact cameras can record 120 or 240 fps at modest resolutions, useful for low-budget music videos and social content.
• Dedicated high-speed cinema cameras, often called "Phantom-class" after a well-known line, record roughly 1,000 to 2,000 fps at high definition or 4K, and many thousands of frames per second at reduced resolution. They record to internal memory in short bursts and then offload to storage.
• Scientific and industrial high-speed cameras reach tens or hundreds of thousands of frames per second at very small resolutions, used for extreme effects like bullets or explosions.

Dedicated high-speed cameras usually hold only a few seconds of footage in their internal memory at maximum rate. A camera with 64 GB of memory at 1,000 fps at 2K might hold under ten seconds of recording. This short window is why triggering matters so much.

Shutter, exposure and the light problem

At normal frame rates with a 180-degree shutter, the exposure time per frame is half the frame interval: 1/48 second at 24 fps. At high frame rates, the exposure time becomes very short.

Here \( \theta \) is the shutter angle and \( f \) the frame rate. At 1,000 fps with a 180-degree shutter, \( t_{\text{exp}} = 0.5 / 1000 = 1/2000 \) s.

Shorter exposure means less light reaches the sensor for each frame. The number of stops of light lost compared with normal speed is:

Because of the light requirement, high-speed tabletop sets are among the brightest places in filmmaking. Large HMIs, powerful LED fixtures and tungsten sources are placed close to small products. Heat can melt food, warp plastic and make talent uncomfortable, so cool LED sources are popular, and heat-sensitive subjects are kept out of the light until the take.

Flicker at high frame rates

Many light sources flicker. Mains electricity alternates at 60 Hz in North America, and many sources pulse twice per cycle, at 120 Hz. At normal frame rates with a shutter synchronized to the mains, this flicker is invisible. At high frame rates, each frame captures a different part of the light's cycle, so brightness varies from frame to frame and the footage pulses visibly.

• Tungsten sources have thermal inertia, so their filaments cool only slightly between pulses. They flicker less but still show some variation at very high frame rates, especially small, low-wattage bulbs.
• HMI sources need flicker-free electronic ballasts, often with a special high-speed mode at high frequency.
• LED sources vary: some use pulse-width modulation dimming, which flickers at frequencies that may show at high speed. Fixtures marketed for high-speed work use constant-current drivers or very high PWM frequencies.
• Fluorescent tubes and practical lights in locations often flicker badly and must be tested or replaced.
• Daylight does not flicker, which makes it ideal when it is strong enough.

Testing is essential. Before a high-speed day, the camera team shoots each fixture at the planned frame rate and dimmer settings and watches for pulsing. Some fixtures are flicker-free only at full output; dimming them can introduce flicker.

Triggering and timing

High-speed cameras record in a loop. They continuously fill their memory and overwrite old frames until the operator presses a trigger. The trigger can be set to save frames before it, after it or around it.

• Pre-trigger: the camera keeps the frames recorded before the trigger. Useful when the operator reacts after an event, such as a balloon bursting.
• Post-trigger: the camera records after the trigger, useful when the event starts on cue.
• Centre trigger: the camera keeps some frames before and some after, a common default.

For repeatable effects, the action itself is triggered electronically. A solenoid drops a strawberry, a pneumatic ram fires a splash, or a robot arm throws liquid, all synchronized with the camera trigger so every take is timed to the millisecond. Laser beams or sound triggers can also start recording when something crosses a line.

Data, offload and workflow

After each take, the high-speed camera must offload its memory to storage before it can record again at full speed. Offloading can take a minute or more, which affects the rhythm of the day. Some cameras let the operator review the take and save only the useful part, which saves time and storage.

• Plan the number of takes and the offload time in the schedule.
• Trim takes to the useful frames before offloading when the camera allows it.
• Use fast media and a dedicated data wrangler or digital imaging technician.
• Record metadata: frame rate, shutter, trigger position and take notes.
• Review every take at playback speed with the director and client before moving on.

Lighting and art direction for slow motion

Slow motion reveals details that real-time viewing hides. Every dust particle, smudge, stray hair and wobble becomes visible. High-speed sets therefore need meticulous preparation.

• Clean surfaces and air: dust floating in a backlight becomes an obvious distraction at 1,000 fps.
• Plan movement: hair, fabric and liquids need wind, rigs and rehearsals to move beautifully.
• Light for the moment of action rather than the static frame; a splash crown may rise into an area that is dark.
• Use backlight and edge light to define fast-moving liquids and particles.
• Control the background so that motion reads clearly, often with dark or plain backgrounds.

The high-speed crew

Specialist high-speed cameras usually come with a technician from the rental house, who operates the camera system, manages triggers, offloads data and troubleshoots. The director of photography decides frame rate, exposure and composition; the technician makes sure the system works. On large jobs, effects rigs are built by a special effects team, and a robot arm operator may also be present (Lesson 5).