{"course":{"slug":"motion-and-audio","title":"Motion, Sound and Music","url":"https://addaly.com/learn/motion-and-audio","level":"intermediate","track":"create"},"lesson":{"slug":"frame-rate-and-cadence","title":"Frame rates, shutter and judder","url":"https://addaly.com/learn/motion-and-audio/frame-rate-and-cadence","position":43,"of":72,"minutes":10,"answer":"Motion reads as continuous because each frame carries blur covering roughly half the interval to the next (the 180-degree rule) and judder comes from showing frames for unequal lengths of time.","keyIdea":"Motion reads as continuous because each frame carries blur covering roughly half the interval to the next (the 180-degree rule) and judder comes from showing frames for unequal lengths of time, as 3:2 pulldown does when 24 is forced onto a 60Hz display."},"body":{"format":"markdown","text":"## The numbers, and where they come from\n\n24, 25, 30, 50, 60. None of them is arbitrary and the history explains every annoyance that follows.\n\n**24** was the sound-film standard from the late 1920s: the slowest rate that gave acceptable optical soundtrack quality, chosen to save film stock. It remains the cinema rate and the reason \"cinematic\" and \"24fps\" have become entangled.\n\n**25** is European television, one frame per two cycles of 50Hz mains.\n\n**30** is American television, matched to 60Hz mains. When colour was added to NTSC in 1953, the frame rate was nudged to **29.97** to stop the colour subcarrier interfering with the audio carrier. That fractional rate is why American video timing is awkward to this day.\n\n**50 and 60** are the doubled rates, used for sport, for broadcast, and for anything where motion clarity matters more than the look of 24.\n\n## Drop-frame timecode is not dropped frames\n\nAt 29.97fps, one hour of video contains 107,892 frames, while an hour of timecode counting at 30fps expects 108,000. Over an hour, timecode drifts about 3.6 seconds ahead of the clock.\n\n**Drop-frame timecode** fixes this by skipping certain *labels*: the numbers `:00` and `:01` at the start of each minute except every tenth minute. No frames are removed. It is a renumbering scheme so that timecode matches wall-clock time, which matters when a programme has to be exactly 28 minutes 30 seconds.\n\nPeople lose hours to this. Nothing is missing; the counter skips.\n\n## Shutter angle, and what 180 degrees means\n\nA film camera's rotating shutter is a disc with a slice cut out. A 180-degree opening means the film is exposed for half of each frame's duration: at 24fps, 1/48 second.\n\nThis is the **180-degree rule**, and it is a look rather than a law. Each frame carries motion blur covering half the interval between frames, so consecutive frames overlap in what they record and the eye reads continuous movement.\n\n- **Shorter exposure** (90 degrees, 1/96 at 24fps) gives crisp, blur-free frames that strobe visibly in motion. The opening sequence of *Saving Private Ryan* is the famous example.\n- **Longer exposure** (270 or 360 degrees) gives heavy blur and a dreamy, smeared quality.\n\nOn a digital camera it is a shutter speed, and the arithmetic is: **shutter speed = 1 / (2 × frame rate)** for the standard look. 1/50 at 25fps, 1/60 at 30fps, 1/120 at 60fps.\n\nThe practical consequence outdoors: matching 1/50 in bright sunlight needs a very small aperture or a neutral-density filter, which is why ND filters are standard kit rather than an accessory. Going to 1/500 to control exposure instead gives a strobing look you did not choose.\n\n## Judder, and why 24fps looks odd on a 60Hz screen\n\nTo show 24 frames a second on a 60Hz display, something has to be repeated. 60 does not divide by 24, so the standard scheme (**3:2 pulldown**) shows one frame for 3 refreshes and the next for 2, alternating.\n\nThe result is that consecutive frames are displayed for different lengths of time. A smooth pan then advances in uneven increments, which reads as a stutter: **judder**. It is most visible on slow horizontal camera movement, which is why cinematographers have a rule of thumb about how slowly a pan may cross the frame at 24fps before it falls apart.\n\nModern displays running at 120Hz can show 24fps cleanly (120 is divisible by 24, five refreshes each) which is one real reason a film looks different on a high-refresh screen.\n\n## Changing frame rate, three ways\n\n**Conform.** Play the frames at a different rate. 25fps material played at 24 is 4 per cent longer, with correspondingly lower-pitched audio unless it is corrected. This is the traditional film-to-PAL method, in reverse.\n\n**Duplicate or drop frames.** Cheap, and it produces exactly the judder described above.\n\n**Interpolate.** Generate new frames between the real ones by estimating motion. Modern optical-flow retiming is impressive and fails in characteristic ways: around fast-moving thin objects, at occlusion boundaries where something appears from behind something else, and on repetitive textures where the motion estimate is ambiguous. The artefacts look like melting or tearing.\n\nFor slow motion, the honest answer is to shoot at the higher rate. 120fps played at 24 is true five-times slow motion with real frames. Interpolating 24 up to 120 invents four frames out of every five, and on anything with complex motion it shows.\n\n## Mixed rates on one timeline\n\nDropping 60fps material into a 24fps project makes the editor decide what to do, and different editors default differently: some conform to slow motion, some drop frames. This is the source of a great deal of \"why is this clip stuttering\" confusion.\n\nSet the project rate first, deliberately, and convert anything that does not match on the way in rather than letting the timeline improvise.\n\n## Free tools\n\n**ffmpeg** handles all of it:\n\n```bash\n# conform: relabel 25fps as 24fps, no new frames, 4% longer\nffmpeg -i in.mov -r 24 -filter:v \"setpts=25/24*PTS\" out.mov\n\n# interpolate 24 to 60 with motion estimation\nffmpeg -i in.mov -vf \"minterpolate=fps=60:mi_mode=mci\" out.mov\n```\n\n**DaVinci Resolve**'s free edition has optical-flow retiming that is good. **Blender**'s video sequencer and **Kdenlive** both do frame-rate conversion. **RIFE** and similar open-source interpolation models often beat classical optical flow on difficult motion and are free to run locally.\n\n## The limitation\n\nThere is no correct frame rate, only conventions and what an audience is used to. The reaction to *The Hobbit* at 48fps (that it looked like television or a stage play rather than a film) is the clearest evidence that frame rate carries learned meaning rather than an objective quality ranking. Higher is not better; it is different, and the difference is cultural as much as perceptual.\n\nInterpolation also has a hard limit that no model removes: information absent from the source cannot be recovered, only invented plausibly. When the invention is wrong it is wrong in a way that draws attention, which is why broadcast and cinema still prefer to shoot at the rate they intend to show."},"check":{"question":"A director wants true slow motion in a 24fps project. One editor proposes shooting at 120fps and conforming; another proposes shooting at 24fps and interpolating up. What is the substantive difference?","options":[{"text":"Interpolation preserves the 180-degree shutter relationship while conforming from 120fps does not, so the interpolated version will look more natural in motion.","correct":false,"tempting":"Inverts the shutter argument: 120fps footage has a shorter exposure per frame, which is exactly why real high-speed footage looks crisp."},{"text":"Conforming from 120fps changes the audio pitch and requires a separate sound pass, whereas interpolation leaves the audio untouched, which is the main practical distinction between the two approaches.","correct":false,"tempting":"Picks a real but minor consequence; slow-motion inserts rarely carry sync sound in the first place."},{"text":"Interpolation has to be done at the source resolution while conforming can be done at any resolution, so the 120fps route allows a higher-quality master.","correct":false,"tempting":"Invents a resolution constraint; both approaches operate at whatever resolution the footage was captured at."},{"text":"The 120fps route has five genuinely photographed frames per source frame, while interpolation invents four of every five and fails at occlusion boundaries and on repetitive texture.","correct":true}]},"prev":{"slug":"log-and-luts","title":"Log footage and what a LUT is not","url":"https://addaly.com/learn/motion-and-audio/log-and-luts"},"next":{"slug":"exporting-without-a-huge-file","title":"Your hero video is why the page feels slow","url":"https://addaly.com/learn/motion-and-audio/exporting-without-a-huge-file"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'Frame rates, shutter and judder', in Motion, Sound and Music. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/motion-and-audio/frame-rate-and-cadence (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"Frame rates, shutter and judder","courseTitle":"Motion, Sound and Music","module":{"index":5,"slug":"codecs-colour-and-delivery","title":"Codecs, colour and delivery"},"answer":null,"keyIdea":"Motion reads as continuous because each frame carries blur covering roughly half the interval to the next (the 180-degree rule) and judder comes from showing frames for unequal lengths of time, as 3:2 pulldown does when 24 is forced onto a 60Hz display.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"the-numbers-and-where-they-come-from","runs":[{"text":"The numbers, and where they come from"}]},{"t":"p","runs":[{"text":"24, 25, 30, 50, 60. None of them is arbitrary and the history explains every annoyance that follows."}]},{"t":"p","runs":[{"text":"24","b":true},{"text":" was the sound-film standard from the late 1920s: the slowest rate that gave acceptable optical soundtrack quality, chosen to save film stock. It remains the cinema rate and the reason “cinematic” and “24fps” have become entangled."}]},{"t":"p","runs":[{"text":"25","b":true},{"text":" is European television, one frame per two cycles of 50Hz mains."}]},{"t":"p","runs":[{"text":"30","b":true},{"text":" is American television, matched to 60Hz mains. When colour was added to NTSC in 1953, the frame rate was nudged to "},{"text":"29.97","b":true},{"text":" to stop the colour subcarrier interfering with the audio carrier. That fractional rate is why American video timing is awkward to this day."}]},{"t":"p","runs":[{"text":"50 and 60","b":true},{"text":" are the doubled rates, used for sport, for broadcast, and for anything where motion clarity matters more than the look of 24."}]},{"t":"h2","id":"drop-frame-timecode-is-not-dropped-frames","runs":[{"text":"Drop-frame timecode is not dropped frames"}]},{"t":"p","runs":[{"text":"At 29.97fps, one hour of video contains 107,892 frames, while an hour of timecode counting at 30fps expects 108,000. Over an hour, timecode drifts about 3.6 seconds ahead of the clock."}]},{"t":"p","runs":[{"text":"Drop-frame timecode","b":true},{"text":" fixes this by skipping certain "},{"text":"labels","i":true},{"text":": the numbers "},{"text":":00","code":true},{"text":" and "},{"text":":01","code":true},{"text":" at the start of each minute except every tenth minute. No frames are removed. It is a renumbering scheme so that timecode matches wall-clock time, which matters when a programme has to be exactly 28 minutes 30 seconds."}]},{"t":"p","runs":[{"text":"People lose hours to this. Nothing is missing; the counter skips."}]},{"t":"h2","id":"shutter-angle-and-what-180-degrees-means","runs":[{"text":"Shutter angle, and what 180 degrees means"}]},{"t":"p","runs":[{"text":"A film camera’s rotating shutter is a disc with a slice cut out. A 180-degree opening means the film is exposed for half of each frame’s duration: at 24fps, 1/48 second."}]},{"t":"p","runs":[{"text":"This is the "},{"text":"180-degree rule","b":true},{"text":", and it is a look rather than a law. Each frame carries motion blur covering half the interval between frames, so consecutive frames overlap in what they record and the eye reads continuous movement."}]},{"t":"ul","items":[[{"text":"Shorter exposure","b":true},{"text":" (90 degrees, 1/96 at 24fps) gives crisp, blur-free frames that strobe visibly in motion. The opening sequence of "},{"text":"Saving Private Ryan","i":true},{"text":" is the famous example."}],[{"text":"Longer exposure","b":true},{"text":" (270 or 360 degrees) gives heavy blur and a dreamy, smeared quality."}]]},{"t":"p","runs":[{"text":"On a digital camera it is a shutter speed, and the arithmetic is: "},{"text":"shutter speed = 1 / (2 × frame rate)","b":true},{"text":" for the standard look. 1/50 at 25fps, 1/60 at 30fps, 1/120 at 60fps."}]},{"t":"p","runs":[{"text":"The practical consequence outdoors: matching 1/50 in bright sunlight needs a very small aperture or a neutral-density filter, which is why ND filters are standard kit rather than an accessory. Going to 1/500 to control exposure instead gives a strobing look you did not choose."}]},{"t":"h2","id":"judder-and-why-24fps-looks-odd-on-a-60hz-screen","runs":[{"text":"Judder, and why 24fps looks odd on a 60Hz screen"}]},{"t":"p","runs":[{"text":"To show 24 frames a second on a 60Hz display, something has to be repeated. 60 does not divide by 24, so the standard scheme ("},{"text":"3:2 pulldown","b":true},{"text":") shows one frame for 3 refreshes and the next for 2, alternating."}]},{"t":"p","runs":[{"text":"The result is that consecutive frames are displayed for different lengths of time. A smooth pan then advances in uneven increments, which reads as a stutter: "},{"text":"judder","b":true},{"text":". It is most visible on slow horizontal camera movement, which is why cinematographers have a rule of thumb about how slowly a pan may cross the frame at 24fps before it falls apart."}]},{"t":"p","runs":[{"text":"Modern displays running at 120Hz can show 24fps cleanly (120 is divisible by 24, five refreshes each) which is one real reason a film looks different on a high-refresh screen."}]},{"t":"h2","id":"changing-frame-rate-three-ways","runs":[{"text":"Changing frame rate, three ways"}]},{"t":"p","runs":[{"text":"Conform.","b":true},{"text":" Play the frames at a different rate. 25fps material played at 24 is 4 per cent longer, with correspondingly lower-pitched audio unless it is corrected. This is the traditional film-to-PAL method, in reverse."}]},{"t":"p","runs":[{"text":"Duplicate or drop frames.","b":true},{"text":" Cheap, and it produces exactly the judder described above."}]},{"t":"p","runs":[{"text":"Interpolate.","b":true},{"text":" Generate new frames between the real ones by estimating motion. Modern optical-flow retiming is impressive and fails in characteristic ways: around fast-moving thin objects, at occlusion boundaries where something appears from behind something else, and on repetitive textures where the motion estimate is ambiguous. The artefacts look like melting or tearing."}]},{"t":"p","runs":[{"text":"For slow motion, the honest answer is to shoot at the higher rate. 120fps played at 24 is true five-times slow motion with real frames. Interpolating 24 up to 120 invents four frames out of every five, and on anything with complex motion it shows."}]},{"t":"h2","id":"mixed-rates-on-one-timeline","runs":[{"text":"Mixed rates on one timeline"}]},{"t":"p","runs":[{"text":"Dropping 60fps material into a 24fps project makes the editor decide what to do, and different editors default differently: some conform to slow motion, some drop frames. This is the source of a great deal of “why is this clip stuttering” confusion."}]},{"t":"p","runs":[{"text":"Set the project rate first, deliberately, and convert anything that does not match on the way in rather than letting the timeline improvise."}]},{"t":"h2","id":"free-tools","runs":[{"text":"Free tools"}]},{"t":"p","runs":[{"text":"ffmpeg","b":true},{"text":" handles all of it:"}]},{"t":"code","lang":"bash","label":"Bash","text":"# conform: relabel 25fps as 24fps, no new frames, 4% longer\nffmpeg -i in.mov -r 24 -filter:v \"setpts=25/24*PTS\" out.mov\n\n# interpolate 24 to 60 with motion estimation\nffmpeg -i in.mov -vf \"minterpolate=fps=60:mi_mode=mci\" out.mov"},{"t":"p","runs":[{"text":"DaVinci Resolve","b":true},{"text":"’s free edition has optical-flow retiming that is good. "},{"text":"Blender","b":true},{"text":"’s video sequencer and "},{"text":"Kdenlive","b":true},{"text":" both do frame-rate conversion. "},{"text":"RIFE","b":true},{"text":" and similar open-source interpolation models often beat classical optical flow on difficult motion and are free to run locally."}]},{"t":"h2","id":"the-limitation","runs":[{"text":"The limitation"}],"role":"limit"},{"t":"p","runs":[{"text":"There is no correct frame rate, only conventions and what an audience is used to. The reaction to "},{"text":"The Hobbit","i":true},{"text":" at 48fps (that it looked like television or a stage play rather than a film) is the clearest evidence that frame rate carries learned meaning rather than an objective quality ranking. Higher is not better; it is different, and the difference is cultural as much as perceptual."}]},{"t":"p","runs":[{"text":"Interpolation also has a hard limit that no model removes: information absent from the source cannot be recovered, only invented plausibly. When the invention is wrong it is wrong in a way that draws attention, which is why broadcast and cinema still prefer to shoot at the rate they intend to show."}]}],"check":{"question":[{"text":"A director wants true slow motion in a 24fps project. One editor proposes shooting at 120fps and conforming; another proposes shooting at 24fps and interpolating up. What is the substantive difference?"}],"options":[{"text":[{"text":"Interpolation preserves the 180-degree shutter relationship while conforming from 120fps does not, so the interpolated version will look more natural in motion."}],"correct":false,"tempting":[{"text":"Inverts the shutter argument: 120fps footage has a shorter exposure per frame, which is exactly why real high-speed footage looks crisp."}]},{"text":[{"text":"Conforming from 120fps changes the audio pitch and requires a separate sound pass, whereas interpolation leaves the audio untouched, which is the main practical distinction between the two approaches."}],"correct":false,"tempting":[{"text":"Picks a real but minor consequence; slow-motion inserts rarely carry sync sound in the first place."}]},{"text":[{"text":"Interpolation has to be done at the source resolution while conforming can be done at any resolution, so the 120fps route allows a higher-quality master."}],"correct":false,"tempting":[{"text":"Invents a resolution constraint; both approaches operate at whatever resolution the footage was captured at."}]},{"text":[{"text":"The 120fps route has five genuinely photographed frames per source frame, while interpolation invents four of every five and fails at occlusion boundaries and on repetitive texture."}],"correct":true,"tempting":null}],"order":[1,0,3,2],"why":null},"prev":{"slug":"log-and-luts","title":"Log footage and what a LUT is not","minutes":11},"next":{"slug":"exporting-without-a-huge-file","title":"Your hero video is why the page feels slow","minutes":10}}}