Home Cinema · Explainer ·
Why do camera pans stutter on your TV? Film judder, explained
Films run at 24 frames a second, but a 60 Hz TV can't hold 24 frames evenly, so it shows them for three refreshes, then two, then three. That uneven beat is judder, the small hitch you see in slow pans.
Watch the stumble in a simulation, then see why 120 Hz screens, 24p output and 50 Hz speed-up each behave differently, and which setting on your player or TV is worth changing first.
You are watching a slow pan across a city skyline. The picture is sharp, the colours are right, and yet the buildings seem to hitch as they slide past, as if the camera were catching on something. Nothing is broken. This is judder, and it comes from a simple piece of arithmetic: films are made at 24 frames a second, and most screens do not refresh at a multiple of 24 that they can divide evenly in every situation.
The arithmetic that goes wrong
Cinema settled on 24 frames a second when sound arrived in the late 1920s, and the rate stuck. A television refreshes on its own clock. A 60 Hz screen refreshes 60 times a second, and 60 divided by 24 is 2.5. You cannot show every film frame for two and a half refreshes, so the screen alternates: one frame for three refreshes, the next for two, then three, then two. This is called 3:2 pulldown. Over one second it adds up exactly (12 frames shown three times, 12 shown twice, 60 refreshes in all), but the frames are held for uneven lengths of time, and on a steady camera move your eye reads that unevenness as a small stumble, over and over.
Screens that refresh at 120 Hz do not have the problem: 120 divided by 24 is exactly 5, so every frame is held for the same five refreshes. Screens and players that can switch to a true 24 Hz output (the usual name is 24p) hold every frame for the same time too. In many European countries the mains-linked TV standard is 50 Hz, where older DVDs and some broadcasts solve the mismatch by simply running the film at 25 frames a second, about 4% fast: a two-hour film then lasts roughly an hour and 55 minutes.
See the stumble
Pick a screen type. The dot is a camera pan across the frame; the strip below shows which film frame (A, B, C, D…) the screen shows on each refresh. This is a simulation, so your own screen's real refresh rate will not change what you see.
What to change
The fix is to let the picture run at a rate that divides evenly into 24, and the exact menu names differ by brand and model. The ideas to look for are these:
- Set your Blu-ray player or streaming box to 24p, or to "match frame rate" or "source direct". The player then asks the TV to switch to a 24 Hz-friendly mode for films and back again afterwards. If it is left on a fixed 60 Hz output, you get 3:2 pulldown.
- Check that the TV accepts 24p. Most current sets do, and 120 Hz panels handle it cleanly. Some projectors and older sets convert it back to 60 Hz anyway, and then nothing changes.
- Do not reach for motion smoothing first. It hides judder by inventing in-between frames, and gives the film the polished, home-video look that many people dislike. A "low" or "film" judder setting, where your TV has one, is a gentler compromise.
- Remember that some of it is just 24 frames. A very fast pan on a bright scene looks choppy in a cinema too. An even cadence makes a pan steady, not video-game smooth.
If you only change one thing, change the output setting. It costs nothing, and after you have seen a pan with an even beat it is hard to unsee the uneven one.
Where this comes from
The cadence explanations here follow RTINGS's guides to motion cadence and 24p playback on TVs. The 50 Hz speed-up is the long-established PAL practice described in Wikipedia's 576i article. Settings are named differently on every set, so check your own manual.