When a helicopter in motion is captured on video, what we saw is a phenomenon known as aliasing whereby we a general blur along with occasional pronounced blades that appeared to be rotating in alternating directions. However, if you managed to capture the footage at a frame rate exactly the same revolution per minute of the spinning object, such as in the example of a helicopter, it will appear as though the blades aren’t moving at all.
I know, it is hard to wrap your head around this concept, but you don’t need to torment your brain over this because cameraman Chris Fay has a video that perfectly demonstrate this phenomena. Here’s the gist of how it works from the horse’s mouth:
“I shot the Video using my Samsung S7 Smartphone set on „Pro“ Function, where you can adjust the ISO and the shutter speed manually, so you have full control. To get rid of the motion blur you have to set the shutter speed to a very high value. The Original Video is captured in QHD, which is 2560 x 1440 Pixels on the S7.
The frame rate was 30 fps. I am guessing that the helicopter had a rotation speed of about 6 rounds per seconds and as the 30 fps in the camera is a multiple of that, the blades do look like they are not moving. The helicopter does have the same RPM speed (rounds per minute) when lifting off, it just adjust the tilt of the blades in order to lift off. This circumstance helped to create this illusion.”
So, there you have it, the weirdest thing that happen when camera shutter speed is perfectly in sync with the helicopter’s rotor speed and rest assure, what you about to see ain’t no animated gif. It is just the science of videography at play.
And here’s the mind-blowing illusion…
Image: screengrab from YouTube video.