WEBVTT - Why Do Helicopter Blades Look Weird On Video?

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<v Speaker 1>Welcome to brain Stuff from How Stuff Works. Hey, brain stuff,

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<v Speaker 1>this is Christian Seger. Have you ever noticed that fast

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<v Speaker 1>spinning objects like helicopter blades and airplane propellers sometimes look

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<v Speaker 1>really weird on film and video. Sure, sometimes you can

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<v Speaker 1>only see a circular blur, but sometimes they appear to

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<v Speaker 1>be spinning very slowly. Sometimes they also look like they're

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<v Speaker 1>spinning backward. In very rare cases, they can even look

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<v Speaker 1>like they're holding still. So what is up with that? Well,

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<v Speaker 1>when you watch a film or a video, nothing on

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<v Speaker 1>screen is actually moving. Instead, you're seeing a succession of

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<v Speaker 1>still images that come so rapidly that your eyes and

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<v Speaker 1>brain interpret them as continuous motion. With this in mind,

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<v Speaker 1>we need to look at two different facts about how

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<v Speaker 1>those images are captured, shutter speed and frame rate. Shutter

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<v Speaker 1>speed is a measure of how along the camera spends

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<v Speaker 1>collecting light each time it takes a picture. The longer

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<v Speaker 1>the film or digital pixel array spends gathering light, the

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<v Speaker 1>more motion blur we see in the image. Let's say

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<v Speaker 1>you want to shoot a video of yourself doing some

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<v Speaker 1>rad throwing knife tricks. If you toss a knife at

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<v Speaker 1>ten ms per second and film it at a shutter

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<v Speaker 1>speed of one quarter of a second, the knife will

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<v Speaker 1>travel two point five meters while the camera is exposing,

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<v Speaker 1>each frame, coming out as a streaky blur in the

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<v Speaker 1>final video. But if you shoot the same knife with

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<v Speaker 1>a shutter speed of one one of a second, it

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<v Speaker 1>will only travel one centimeter while each picture is taking.

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<v Speaker 1>Beat it will only travel one centimeter while each picture

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<v Speaker 1>is taken, meaning the knife will look less blurry in

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<v Speaker 1>each frame. Now, the same applies to helicopter blades. Long

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<v Speaker 1>exposures will make the blades look more uniformly blurry. At

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<v Speaker 1>quicker shutter speeds, strange looking patterns or even discreet individual

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<v Speaker 1>blades will begin to appear. The second main factor to

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<v Speaker 1>consider is frame rate. We can start with a slow

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<v Speaker 1>motion analogy. Imagine you've got a sun dial. The shadow

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<v Speaker 1>on the dial makes one complete revolution every twenty four hours. Now,

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<v Speaker 1>let's say you take a picture of that sun dial

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<v Speaker 1>exactly once every minute. If you play those pictures in

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<v Speaker 1>order as a video, you'll see the shadows spinning rapidly

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<v Speaker 1>around the clock in the normal clockwise direction. If you

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<v Speaker 1>capture a frame exactly once every twenty four hours in

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<v Speaker 1>this video, the shadow will appear to stand still, and

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<v Speaker 1>if you take the picture once every twenty three hours

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<v Speaker 1>and fifty nine minutes, the dial in the video will

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<v Speaker 1>appear too slowly creep backward. The same principle is at

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<v Speaker 1>work in videos of helicopter blades, only complicated by greater

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<v Speaker 1>speed in both the rotation and the frame rate, and

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<v Speaker 1>by adding more blades. Frame rate is usually expressed in

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<v Speaker 1>frames per second, or FPS. Imagine you're shooting a twenty

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<v Speaker 1>four FPS video of a helicopter rotor that spins one

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<v Speaker 1>full rotation every second. In the video, each revolution will

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<v Speaker 1>thus be broken into twenty four frames. You will see

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<v Speaker 1>the blades rotating normally, just moving one of their full

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<v Speaker 1>rotation in each frame. But if the blades spin exactly

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<v Speaker 1>twenty four times each second and you're still shooting at

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<v Speaker 1>twenty four frames per second, each full revolution will be

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<v Speaker 1>represented by only one frame. The blades will arrive back

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<v Speaker 1>in their starting place each time the camera captures a frame,

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<v Speaker 1>so they'll look like they're standing still. But what if

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<v Speaker 1>you have blades that spin exactly twenty three times each

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<v Speaker 1>second and you're still shooting at twenty four fps. Each

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<v Speaker 1>frame will capture the blades having just made about nine

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<v Speaker 1>percent of a full rotation, the blades will always be

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<v Speaker 1>just a little bit behind where they are are in

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<v Speaker 1>the previous frame. Thus, in the final video, the blades

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<v Speaker 1>will look like they're spinning backwards. But that's not the

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<v Speaker 1>only way our cameras can trick us. Sometimes propellers and

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<v Speaker 1>helicopter blades caught on video can look s shaped or

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<v Speaker 1>even fragmented. This type of distortion is caused by the

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<v Speaker 1>method of pixel capture that's used in digital cameras. Most

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<v Speaker 1>digital video cameras today don't expose the whole frame all

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<v Speaker 1>at once, but instead sample a single line of pixels

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<v Speaker 1>at a time and update the frame line by line.

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<v Speaker 1>This is called a rolling shutter. With a rolling shutter,

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<v Speaker 1>any object moving extremely fast will be sampled in a

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<v Speaker 1>way that distorts its shape across the frame, leading to

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<v Speaker 1>spinning blades that look bent or broken, or appear to

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<v Speaker 1>be hovering separate from the aircraft. In fact, you can

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<v Speaker 1>try this out on your phone's camera by panning quickly

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<v Speaker 1>back and forth while you're taking a video. If your

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<v Speaker 1>camera uses a roll links shutter, the picture will be

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<v Speaker 1>distorted so that solid objects will appear to bend, like

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<v Speaker 1>rubber or Jello as you rotate the lens. Check out

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<v Speaker 1>the brainstuff channel on YouTube, and for more on this

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<v Speaker 1>and thousands of other topics, visit how stuff works dot com.