Frame rate, the number of frames per second, is one of the first settings to choose before a shoot and one of the hardest to fix afterwards. Unlike resolution, which can be scaled down with good results, changing the frame rate of footage already shot almost always introduces judder, duplicated frames or artificial-looking images. It is therefore worth understanding where the common values come from and which to choose for each destination.
Where the numbers come from
24 fps: cinema
Sound cinema settled on 24 frames per second, a compromise between perceived smoothness and the cost of film stock. Ever since, 24 fps has been associated with the «cinematic» look: slightly fragmented motion, with a trail of blur the eye associates with films.
25 and 50: Europe
European analogue television, with the PAL system and a 50 Hz power grid, adopted 50 fields per second, corresponding to 25 full frames. In Italy and most of Europe, television formats are still based on 25 and 50 fps today.
30, 60 and the odd numbers
In the United States and Japan the power grid runs at 60 Hz and the NTSC standard worked at 30 frames per second. When colour was introduced, the rate was lowered slightly, for reasons of signal compatibility, to 30/1.001, roughly 29.97 fps. This is also where 23.976 and 59.94 come from: 24 and 60 divided by 1.001.
| Value | Origin | Typical use today |
|---|---|---|
| 23.976 / 24 | cinema and its NTSC conversion | films, series, narrative content |
| 25 | European PAL | European TV, web video in Europe |
| 29.97 / 30 | NTSC | North American TV, many smartphones |
| 50 | high-motion PAL | sport, European live TV |
| 59.94 / 60 | high-motion NTSC | sport, video games, screen captures |
The shutter matters as much as the frames
How motion looks depends not only on frame rate but also on the exposure time of each frame. The traditional rule, known as the «180-degree shutter», suggests a time of about half a frame's duration: 1/50 of a second at 25 fps, 1/48 or 1/50 at 24 fps, 1/120 at 60 fps.
- With a shorter time each frame is sharp but motion looks choppy, an effect typical of action scenes or of footage shot in full sun without ND filters.
- With a longer time motion becomes soft and smeared, to the point of looking mushy.
Light flicker
Mains-powered lights, especially cheap LEDs and fluorescent tubes, pulse at a frequency tied to the power grid. In Europe, with a 50 Hz grid, shooting at 30 or 60 fps with non-matching exposure times can produce dark bands rolling across the picture. This is why in Italy it is safer to shoot at 25 or 50 fps with times of 1/50 or 1/100, unless you fully control the lighting.
Which frame rate to choose
- Narrative video, interviews, documentaries: 24 or 25 fps.
- Content for European television: 25 or 50 fps.
- Sport, fast motion, video games, tutorials with screen scrolling: 50 or 60 fps.
- Slow motion: shoot at 100, 120 or more and play back at 25 or 24 fps, for a four- or five-fold slowdown.
The most important thing is consistency: all footage in a project should share the same frame rate, or an exact multiple, such as 25 and 50.
Converting between frame rates
Mixing different cadences is the main cause of irregular motion. The possible techniques are:
- duplicating or dropping frames: simple, but produces regular stutter, clearly visible in pans;
- frame blending: neighbouring frames are mixed, with a double-image effect;
- motion-estimated interpolation: the software creates in-between frames; it works well on simple motion, but generates artefacts on complex edges and overlapping objects;
- speed change: a 24 fps film is played at 25, speeding everything up by about 4%; this is the method historically used for films on European TV, and it also requires correcting the audio pitch.
With FFmpeg a simple conversion is done with the fps filter, while interpolation is available through the minterpolate filter, which is slow and should be tested on an excerpt.
Smartphones and variable frame rate
Many smartphones record with a variable frame rate: in low light they slow the cadence to expose for longer. Some editing programs handle these files badly, with audio drifting out of sync. Converting the file to a constant frame rate first, or setting the phone to a fixed value, avoids the problem.
In short
- 24 fps comes from cinema, 25 and 50 from European TV, 29.97 and 59.94 from NTSC.
- Exposure time affects how motion looks as much as frame rate does.
- In Europe, 25 or 50 fps reduce flicker from mains lighting.
- Keep the same frame rate throughout a project.
- Convert variable frame rate files before editing.
