Here is the Valid Answer.
If you’re a regular reader, you know that I am on the record with a deep belief that photography and filmmaking are not all about speeds and feeds or how big your megapixels are compared to mine. Nonetheless, I do have a healthy respect for the technical side of the craft — and for those who dive in deep.
Like the guy who made this video, for example.
This optical illusion is “purportedly” made possible by synching the camera shutter speed with the rotation of the helicopter’s blades, giving the latter the appearance of “staticity.” Some cry hoax. Others say it’s real. Those who believe it is real have engaged in lengthy debate about how it was achieved. The two sides’ arguments break down like this:
SS: “As the title of the video suggests, the filmmaker synched his shutter speed with the rotation of the helicopter blades to make it appear as it does.”
FR: “This is a matter of frame rate, not shutter speed. The frame rate has to be synched such that with each frame exposure the blades are in the exact same position.”
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 let’s 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.
Courtesy : Chasejarvis , Petapixel