Why can a camera make a straight moving object look bent?

A fast-moving propeller can look strangely curved in a photograph even when the blades are straight. One cause is a rolling shutter: different rows of the image sensor begin and end their exposures at slightly different times.

Each row can capture a sharp slice of the object, yet those slices need not belong to the same instant. When the camera assembles them into one frame, a moving edge may occupy a different horizontal position in each successive row. The resulting edge looks slanted or curved.

Consider an invented five-row sensor that starts rows 2 milliseconds apart. The last row starts 8 milliseconds after the first. If an edge moves 3 image pixels per millisecond, its position shifts by 24 pixels between those row starts. The diagram turns that timing difference into a visible slope.

Shortening each row’s exposure reduces motion blur, but it does not by itself eliminate the delay between row start times. A global shutter exposes all rows over the same interval, while faster rolling readout reduces the offset. These are different design choices, not just settings for a sharper photograph.

The image is a record of measurements distributed over time. Understanding when each measurement occurred can explain an apparently impossible shape without imagining that the real object bent.