Rolling shutter is the answer to why concrete bends, propellors break up, and trees turn to jelly when you’re filming them while either you, or the object, is moving quickly in front of certain cameras.
But what exactly is it? This 7-minute video by SmarterEveryDay might just be one of the best explanations for the phenomenon on the internet.
In the video, Destin points out that your phone’s sensor is scanned a number of times when taking a video. This is an electronic shutter, but film cameras would suffer the same with their spinning semi-circle shutter shapes.
As the “scanning line” moves down the sensor, the image is recorded and gradually created. Obviously this happens at high speed, but if what you’re photographing is moving past quickly, it can have a weird effect.
You see that propeller splitting up and breaking into strange lines floating through the air? Why does this happen? By analyzing shots of propellers shot on an iPhone with shots taken by a high-speed camera, it’s possible to see exactly what’s going on. The green line simulates a single event of your phone scanning across the sensor and recording the image as it moves.
Now that makes a lot more sense, don’t you think?
P.S. If you need an even more heady explanation of this, here’s a deeper dive into the math behind the rolling shutter effect.