LESSON 91
Slow motion
Shoot fast, play normal speed.
IN THIS LESSON
- How slow motion works
- Three ways to get it
- Working out the numbers
- Practice: getting clean slow motion
How slow motion works
Slow motion makes time appear to slow down. The classic method is to capture frames much faster than they will be played back, then show them at the normal rate. Film-makers call this overcranking, from the days when a camera operator turned the handle faster than usual. If you record at 120 frames a second and play at 24, each second of action lasts five seconds on screen. The effect is credited to the Austrian priest August Musger in the early twentieth century, and it is now everywhere from sport replays to action cinema.
Three ways to get it
There are three main routes. First, high-speed capture, which records many real frames and gives the cleanest result. Second, simply playing normal footage slower, which is common for television replays but looks jerky because frames are repeated. Third, software that invents new frames between the real ones, called time-stretching or frame interpolation. The routes can be combined, for example by interpolating between overcranked frames to slow motion even further. Speed ramping, as in the bullet-dodging scenes of The Matrix, changes the capture rate during a shot.
Working out the numbers
The slow-down factor is the capture rate divided by the playback rate. At a 25 fps timeline, 50 fps is half speed, 100 fps is a quarter speed and 250 fps is a tenth. The table below shows a few common pairs, worked out from that simple division. Remember that faster capture means shorter exposures: to keep a 180-degree look at 100 fps you need 1/200 of a second, which costs two stops of light compared with 1/50. High-speed modes on many cameras also crop the sensor or lower the resolution.
Practice: getting clean slow motion
This is working practice. Give yourself far more light than you think you need, especially indoors, and avoid cheap lamps that flicker, because high frame rates reveal every pulse. Choose subjects with clear motion: water, hair, fabric, a jump, a pour. Frame wider than usual, because slowed action often drifts out of a tight frame. Set your editor's clip interpretation to the timeline rate so the footage plays slowly without being blended. Shoot a few seconds of handles before and after the action, because slowed clips burn through those extra frames quickly and the editor will want room to cut.
| Capture | Playback | Speed on screen |
|---|---|---|
| 50 fps | 25 fps | 1/2 (half speed) |
| 60 fps | 24 fps | 1/2.5 |
| 100 fps | 25 fps | 1/4 |
| 120 fps | 24 fps | 1/5 |
| 240 fps | 24 fps | 1/10 |
Try it today
- 01Set your camera to its highest frame rate and 1/4 of its time as shutter.
- 02Film water pouring into a glass in good daylight.
- 03Put the clip on a 25 fps timeline and conform it to 25.
- 04Compare it with the same action shot at 25 and slowed in software.
- 05Note which looks cleaner, and how much light each needed.