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A bird magically floats because of a camera frame rate trick

posted by Jason Kottke   Jul 19, 2017

You know when you’re watching a fan or a wheel or something else quickly spinning and it seems to stop spinning and even looks like it’s spinning backwards? And you blink your eyes and remind yourself you’re not on drugs and haven’t been drinking heavily but it’s still somehow simultaneously spinning and not? This optical illusion occurs most commonly with video cameras (but can also occur looking through your normal eyeballs) when the frame rate of the camera matches some multiple of the rate of the thing being filmed, as with this magically levitating helicopter.

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 rotar 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.

In the video above, a home security camera catches a bird flying with a wing speed matching the frame rate of the camera, which makes it look like the bird is just magically hanging in the air, like some sort of avian wizard.