Understanding Video Frame Rates

Frame rate is how many still pictures make a second of video. Every rate in common use is there for a historical reason rather than a technical ideal, which is why the list is such an odd collection of numbers.

The common rates

• 24 fps: the film standard for roughly a century. Chosen as the lowest rate that carried sound acceptably, and now inseparable from how audiences read something as cinematic.
• 25 fps: the PAL television standard, matching the 50 Hz mains frequency used across much of the world.
• 30 fps: the NTSC-region equivalent, matching 60 Hz mains.
• 50 and 60 fps: double rates, much clearer on fast motion, standard for sport and gameplay.
• 120 fps and above: usually captured for slow motion rather than played back at that speed.

Where 29.97 came from

When colour was added to black and white television in the NTSC regions, the colour information had to fit alongside the existing signal without interfering with it. The solution involved shifting the frame rate slightly, dividing 30 by 1.001 to give 29.97. Film at 24 got the same treatment, producing 23.976.

That decision from the 1950s is still in every editing application today. A timeline at 29.97 drifts against a real clock by about 3.6 seconds an hour, which matters when a broadcast has to hit an exact duration. Drop frame timecode hides the drift by skipping certain numbers in the count. It skips numbers, never frames: no picture is lost, only labels.

Higher is not automatically better

Sixty frames a second is objectively smoother and often the wrong choice. Audiences associate high frame rates with live broadcast and video games, and low ones with cinema. Drama shot at 60 frequently reads as unconvincing for reasons viewers cannot articulate; it looks like a recording of people rather than a film.

Where high rates clearly help is fast motion that must stay legible: sport, gameplay, anything with rapid camera movement.

Shutter speed goes with it

The convention is a shutter angle of 180 degrees, meaning a shutter speed of roughly double the frame rate: 1/48 at 24 fps, 1/50 at 25, 1/60 at 30. This produces the amount of motion blur people expect. A much faster shutter gives a stuttering, hyper-sharp look, which is a deliberate effect when chosen and a mistake when not.

Changing the rate afterwards

There are two operations here and mixing them up causes most frame rate problems. Conforming leaves the frames alone and plays them at a different rate, so the duration and the speed change together: 24 frames played at 25 runs about four per cent short and four per cent fast. Re-timing keeps the duration and adds or drops frames to fit, which preserves the timing but introduces either judder from duplicated frames or smearing from interpolated ones.

The safest approach is to shoot, edit and deliver at one rate. Where a change is unavoidable, converting between rates that divide cleanly, such as 60 to 30, is far less damaging than converting between 25 and 30.

Shooting for slow motion

Slow motion works by capturing more frames than you play back. Shoot at 60 and play at 24 and everything runs at about forty per cent speed with real frames throughout. Slowing footage that was shot at the delivery rate has no extra frames to use, so software has to invent them, and invented frames fail badly around fast movement and edges.