Judder
Also known as: frame judder, telecine judder
Uneven motion caused by a frame rate that does not divide evenly into the display refresh rate, so some frames are held longer.
Updated
What it means
Judder is an arithmetic problem rather than a camera one. A display refreshing sixty times a second showing twenty-four frames has to hold some of them longer than others, and the unevenness is visible in slow pans.
It is not the same as Stutter. Stutter is a machine failing to keep up; judder happens on hardware that is coping perfectly and simply cannot divide one number by the other.
The classic case is film on television. Twenty-four frames into sixty refreshes is a repeating three-two pattern, which is where the broadcast term came from in the first place.
How it works in practice
Matching rates removes it. Shooting at a Frame rate (fps) the display refreshes at a multiple of means every frame is held for the same time, which is why thirty and sixty are safe choices for the web.
Slow camera movement is where it shows. A fast whip hides the unevenness entirely, so a pan over a landscape is the shot that exposes a mismatch immediately.
Players and televisions try to hide it. Frame blending and motion smoothing insert invented frames, which removes the judder and adds the look most people find worse than the problem it solved.
What to watch out for
Converting frame rates introduces it. Fitting twenty-four frames into a thirty-frame timeline duplicates some of them, and Frame interpolation is the alternative that softens edges instead.
Mixed rates in one edit guarantee it somewhere. A sequence of clips at different rates has to settle on one, so Transcoding before editing is cheaper than discovering it at export.
It gets blamed on the file. A viewer seeing uneven motion assumes a bad encode, where the cause is the combination of their display and the rate the video was finished at, which is nobody fault in particular.
How RawClip handles it
Sequences are rendered at one consistent frame rate rather than inheriting a mixture from the source clips, which removes the common cause of this. What a particular television does with that rate afterwards is outside the control of anybody who made it.