Frame rate, shutter, and the flicker in the light

The number you are stuck with

Frame rate is the most consequential setting nobody checks. Get it wrong and every pan in the film judders slightly, forever, and you will not be able to say why.

The world runs on two families. Where mains electricity is 50 Hz — India, most of Europe, Africa, Australia — video is 25 and 50 fps. Where it is 60 Hz — North America, Japan, parts of South America — it is 29.97 and 59.94. Cinema is 24, and its broadcast cousin is 23.976.

Those fractional numbers are not a mistake. When American television added colour in 1953, the colour subcarrier interfered with the audio at exactly 30 fps, so the whole system was slowed by one part in a thousand. Seventy years later every NTSC-region camera still records 29.97 frames per second, and every editor still has to think about it.

Why the mains frequency decides your frame rate

Most artificial light does not shine steadily. Fluorescent tubes, many LED bulbs and almost all cheap LED panels pulse with the mains supply — twice per cycle, so 100 times a second on a 50 Hz supply.

If your shutter interval does not divide evenly into that pulse, each frame catches a slightly different amount of light, and the footage flickers — a slow pulsing of brightness, or on a rolling-shutter sensor, horizontal bands crawling up the frame.

Shoot 25 or 50 fps on 50 Hz mains and the arithmetic works. Shoot 30 fps under 50 Hz lighting and it does not: you get a visible beat. This is why an Indian creator shooting at 30 fps indoors sometimes gets bands and cannot reproduce them outdoors. It is not the camera.

Where you cannot change the frame rate, change the shutter. A shutter speed that is a whole multiple of the flicker period — 1/50 or 1/100 on 50 Hz mains, 1/60 or 1/120 on 60 Hz — cancels it. Some cameras have a flicker-reduction or anti-flicker setting that does this for you.

Shutter angle, and why 180 degrees

Shutter speed controls how long each frame is exposed. Expressed as an angle, it is the fraction of the frame interval the shutter is open: 180° means open for half the interval.

At 25 fps, 180° is 1/50 s. At 50 fps, it is 1/100 s.

This is the default for one reason: it produces about the amount of motion blur people are used to from a century of cinema. Go much faster — 1/500 s on a 25 fps shoot, a 18° shutter — and each frame is sharp, so motion becomes a sequence of crisp stills and pans stutter. That is the Saving Private Ryan look, and it is a choice, not an accident. Go much slower and movement smears.

The editing consequence: sharp-shutter footage cuts badly against normal footage in the same scene, and it is the footage that resists slow motion the worst, because there is nothing to blend between frames.

Mixing frame rates on one timeline

Your timeline has one frame rate. Everything that is not already at it has to be converted, and every conversion is a compromise.

• 30 fps clip on a 25 fps timeline: five frames a second have to go. The editor either drops them, producing a small hitch on every pan, or blends pairs, producing a soft ghost.
• 25 on 30: five frames a second have to be invented, by duplication or blending.
• 50 or 60 on 25 or 30: this one is clean and useful. Every second frame is used and the rest discarded, or — better — you interpret the clip at half speed and get genuine, real-frame slow motion.

The rule that avoids all of it: decide the delivery frame rate before the shoot and set every camera to it. Where you are handed mixed rates, set the timeline to whichever rate the majority of your picture is, and accept the conversion on the minority.

Choosing your project's number

• Uploading to a platform, anywhere: 25 or 30, whichever your region and your lights agree with. Nobody will notice which.
• Wanting slow motion: shoot 50 or 60 and cut on a 25 or 30 timeline.
• Delivering to a broadcaster or festival: they will specify, and they will reject a file that does not match.
• Screen recordings: capture at your timeline's rate. A 60 fps screen capture on a 30 fps timeline makes cursor movement stutter.

In Resolve, the timeline frame rate locks once the first clip lands in a project. To change it you create a new timeline, or change the project setting before importing anything. Kdenlive and Shotcut set it per project in the same way. This is the most common reason somebody's project is at the wrong rate: they never chose, and the first clip chose for them.

Today

Check the frame rate of the last three things you filmed. If they differ, that explains something you have already seen.