Welcome to brain Stuff from How Stuff Works. Hey, brain stuff, this is Christian Seger. Have you ever noticed that fast spinning objects like helicopter blades and airplane propellers sometimes look really weird on film and video. Sure, sometimes you can only see a circular blur, but sometimes they appear to be spinning very slowly. Sometimes they also look like they're spinning backward. In very rare cases, they can even look
like they're holding still. So what is up with that? Well, when you watch a film or a video, nothing on screen is actually moving. Instead, you're seeing a succession of still images that come so rapidly that your eyes and brain interpret them as continuous motion. With this in mind, we need to look at two different facts about how those images are captured, shutter speed and frame rate. Shutter speed is a measure of how along the camera spends
collecting light each time it takes a picture. The longer the film or digital pixel array spends gathering light, the more motion blur we see in the image. Let's say you want to shoot a video of yourself doing some rad throwing knife tricks. If you toss a knife at ten ms per second and film it at a shutter speed of one quarter of a second, the knife will travel two point five meters while the camera is exposing, each frame, coming out as a streaky blur in the
final video. But if you shoot the same knife with a shutter speed of one one of a second, it will only travel one centimeter while each picture is taking. Beat it will only travel one centimeter while each picture is taken, meaning the knife will look less blurry in each frame. Now, the same applies to helicopter blades. Long exposures will make the blades look more uniformly blurry. At quicker shutter speeds, strange looking patterns or even discreet individual
blades will begin to appear. The second main factor to consider is frame rate. We can start with a slow motion analogy. Imagine you've got a sun dial. The shadow on the dial makes one complete revolution every twenty four hours. Now, let's say you take a picture of that sun dial exactly once every minute. If you play those pictures in order as a video, you'll see the shadows spinning rapidly
around the clock in the normal clockwise direction. If you capture a frame exactly once every twenty four hours in this video, the shadow will appear to stand still, and if you take the picture once every twenty three hours and fifty nine minutes, the dial in the video will appear too slowly creep backward. The same principle is at work in videos of helicopter blades, only complicated by greater speed in both the rotation and the frame rate, and
by adding more blades. Frame rate is usually expressed in frames per second, or FPS. Imagine you're shooting a twenty four FPS video of a helicopter rotor that spins one full rotation every second. In the video, each revolution will thus be broken into twenty four frames. You will see the blades rotating normally, just moving one of their full
rotation in each frame. But if the blades spin exactly twenty four times each second and you're still shooting at twenty four frames per second, each full revolution will be represented by only one frame. The blades will arrive back in their starting place each time the camera captures a frame, so they'll look like they're standing still. But what if you have blades that spin exactly twenty three times each
second and you're still shooting at twenty four fps. Each frame will capture the blades having just made about nine percent of a full rotation, the blades will always be just a little bit behind where they are are in the previous frame. Thus, in the final video, the blades will look like they're spinning backwards. But that's not the only way our cameras can trick us. Sometimes propellers and helicopter blades caught on video can look s shaped or
even fragmented. This type of distortion is caused by the method of pixel capture that's used in digital cameras. Most digital video cameras today don't expose the whole frame all at once, but instead sample a single line of pixels at a time and update the frame line by line. This is called a rolling shutter. With a rolling shutter, any object moving extremely fast will be sampled in a way that distorts its shape across the frame, leading to spinning blades that look bent or broken, or appear to
be hovering separate from the aircraft. In fact, you can try this out on your phone's camera by panning quickly back and forth while you're taking a video. If your camera uses a roll links shutter, the picture will be distorted so that solid objects will appear to bend, like rubber or Jello as you rotate the lens. Check out the brainstuff channel on YouTube, and for more on this and thousands of other topics, visit how stuff works dot com.