An Airbus Helicopters H155 lands safely at a Hong Kong heliport - despite all five blades being frozen to a standstill. Or at least that’s the impression we get from this video filmed from a passing ferry by a tourist on a smartphone. Is it a new futuristic flying machine? Or a trailer for the latest James Bond movie?
No, none of the former, it’s simply an optical illusion. The camera shutter speed – that’s the shutter’s rotation speed in front of the sensor - is exactly the same as that of the helicopter’s rotating blades. Therefore, since light coming into the sensor is "cut" at the same speed as the blades, it gives us the impression that the chopper’s main rotor is stationary.
To make it clearer, let’s assume the blades perform one hundred rotations per second. If you shoot the scene with a shutter speed of 1/100th of a second, each blade will have rotated completely and will be in exactly in the same position, hence the impression they’re at a standstill. For proof, watch the helicopter landing at the end of the video, as the blades’ rotation speed decreases. Since the shutter speed remains the same, you start to see them turning again.