A burglary suspect is on the run and the local news is replaying footage of a police helicopter scanning the streets. You turn your full attention to the screen, wanting to know more, when you realize something looks terribly strange. The helicopter blades seem to be spinning ... backward?
Surprisingly, what you are witnessing is not all that uncommon. Fast-spinning objects like helicopter blades, airplane propellers, ceiling fans and even vehicle tire rims, can all look strange on video and film, as host Lauren Vogelbaum explains in the BrainStuff video above. They may appear to be spinning backward, spinning way too slowly or, in rare cases, holding still or bending at odd angles — all when actually spinning in the right direction at a normal speed.
To understand this effect, you've got to know a few things first. Film and video capture a series of still images and play them in fast succession, which our brains interpret as movement. When optical illusion comes into play, it's most likely caused by the way the objects are captured, namely the settings on the shutter speed and frame rate.
Shutter speed is a measure of how long the camera collects light each time it takes a picture. The longer the film or digital pixel array is exposed to light, the more motion-blur will show up in the image. Let's say you're a big fan of knife-throwing competitions and you want to document the contestants' actions.
Let's look at this with a little quick math: If one knife thrower tosses a blade at a speed of 10 meters (32 feet 10 inches) per second, and you film it at a shutter speed where the camera shutter is open for one-quarter of a second, the knife will travel 2.5 meters (8 feet 2 inches) while the camera is exposing each frame. The result? The knife will look like a streaky blur in the video. However, if you shoot that same knife at the same velocity, but use a shutter speed of one one-thousandth of a second, it will only travel one centimeter while each picture is taken. The end result? Crisp and clear footage of a (hopefully) expert knife thrower.
This same knife-throwing analogy applies to spinning blades, like ceiling fans or helicopter blades. Long exposures make the helicopter blades look blurry, while faster shutter speeds will create everything from normal-looking patterns to the appearance of individual blades.
Frame rate-per-second (FPS) also affects the way images appear on film and video. Let's take another look at those helicopter blades. If the blades are spinning exactly 24 times each second, and you're shooting at 24 frames per second, the final image capture will depict blades that appear to be at a standstill. That's because the blades arrive back at their starting position each time the camera captures a frame.
If the FPS is set a little faster than the blade rotation, they'll "almost" make it back to the starting position in each frame, with an end result that looks like they're spinning backward.
The optical illusions don't stop there, either. Sometimes, propellers and helicopter blades can look fragmented or bent, especially when the motion is captured digitally. This is caused by the method of pixel capture. Most digital cameras don't expose the whole frame simultaneously, but instead sample a single line of pixels at a time in a progressive fashion to fill the frame. When objects are moving extremely fast, this rolling shutter captures distorted shapes.
You can try to recreate this effect yourself by taking a video on your phone and quickly panning back and forth. If solid objects look bent in the final product, your phone's camera probably uses a rolling shutter.