Watch: This Is What Happens When You Sync Your Frame Rate with a Helicopter Rotor
No, it's not a magic trick. It's just clever cinematography.
If you perfectly sync your camera's frame rate with the rotation of a helicopter's rotor, some pretty interesting things happen. YouTuber Chris Chris has shared a video that shows a helicopter taking off, which sounds like nothing special, but because the camera that captured the scene had all the right settings, the blades of the aircraft appear to be completely still. Check out this crazy video below:
If you're wondering how this trickery works, it's actually relatively simple. The main source of confusion is whether it's the frame rate or shutter speed that makes the effect possible, but both are actually necessary to pull it off. In an online discussion about the phenomenon, an anonymous reader broke it down:
It’s a factor of both. Since each frame has to ensure the blade is in the same position as the last it therefore needs to be in sync with the rpm of the rotor blades. Shutter speed then needs to be fast enough to freeze the blade without too much motion blur within each frame. Here the rotor has five blades, now lets say the rpm of the rotor is 300. That means, per rotation, a blade is in a specific spot on five counts. That gives us an effective rpm of 1500. 1500rpm / 60secs = 25. Therefore shooting at 25fps will ensure the rotor blades are shot in the same position every frame. Each frame then has to be shot at a fast enough shutter speed to freeze the blade for minimal motion blur.
This weird effect isn't anything new. There are quite a number of videos out there just like this.
And if that wasn't cool enough for you, here's a video that uses the same technique to create the illusion that a stream of water is frozen in time.