What is a rolling shutter, and why do propellers look bent?

Most camera sensors do not capture the whole image at once. They read it out line by line, from top to bottom, over a few milliseconds — and anything moving during that sweep is recorded at a different position in each line, producing distortion.

Rolling shutter. The sensor is read progressively. By the time the bottom row is captured, the scene has moved on from when the top row was. The effects:

Skew. Vertical objects lean when the camera pans quickly — the classic wobbling lamp post in handheld video.

Wobble or jello. Vibration produces a rippling distortion, familiar from footage shot from vehicles.

Bent propellers and rotors. A blade moves substantially between the top and bottom of the frame, so it is recorded as a curve — or, at particular rotation speeds, as several disconnected fragments.

Partial exposure with flash, where only part of the frame is lit.

Banding under artificial light, as mains-frequency flicker illuminates different rows at different points in the cycle.

Global shutter. The entire sensor is captured simultaneously, eliminating all of this. Used in high-end cinema and industrial cameras, and increasingly appearing in consumer sensors.

Why it is not universal. Global shutter historically required additional circuitry per pixel, reducing the light-sensitive area and therefore worsening noise and dynamic range. The engineering trade-off has been genuine, and it is only recently that global shutter sensors have approached rolling shutter sensors on image quality.

What reduces rolling shutter in practice:

Faster readout, which is what "sensor readout speed" refers to in camera reviews and is a meaningful specification.

Stacked sensor designs, which read out far faster.

Lower resolution or line-skipped modes, which read fewer lines.

Mechanical shutters on cameras that have them, which avoid the problem for stills.

Software correction in editing, which estimates and reverses the distortion imperfectly.

The related effect with spinning objects — where a wheel appears to rotate backwards or stand still — is the wagon-wheel effect, caused by frame rate rather than readout, and is a different phenomenon.