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Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain stuff,

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Speaker 1: this is Christian Seger. Have you ever noticed that fast

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Speaker 1: spinning objects like helicopter blades and airplane propellers sometimes look

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Speaker 1: really weird on film and video. Sure, sometimes you can

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Speaker 1: only see a circular blur, but sometimes they appear to

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Speaker 1: be spinning very slowly. Sometimes they also look like they're

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Speaker 1: spinning backward. In very rare cases, they can even look

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

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Speaker 1: when you watch a film or a video, nothing on

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

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Speaker 1: still images that come so rapidly that your eyes and

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Speaker 1: brain interpret them as continuous motion. With this in mind,

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Speaker 1: we need to look at two different facts about how

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Speaker 1: those images are captured, shutter speed and frame rate. Shutter

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Speaker 1: speed is a measure of how along the camera spends

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Speaker 1: collecting light each time it takes a picture. The longer

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Speaker 1: the film or digital pixel array spends gathering light, the

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Speaker 1: more motion blur we see in the image. Let's say

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Speaker 1: you want to shoot a video of yourself doing some

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Speaker 1: rad throwing knife tricks. If you toss a knife at

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Speaker 1: ten ms per second and film it at a shutter

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Speaker 1: speed of one quarter of a second, the knife will

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Speaker 1: travel two point five meters while the camera is exposing,

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Speaker 1: each frame, coming out as a streaky blur in the

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Speaker 1: final video. But if you shoot the same knife with

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Speaker 1: a shutter speed of one one of a second, it

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Speaker 1: will only travel one centimeter while each picture is taking.

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Speaker 1: Beat it will only travel one centimeter while each picture

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Speaker 1: is taken, meaning the knife will look less blurry in

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Speaker 1: each frame. Now, the same applies to helicopter blades. Long

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Speaker 1: exposures will make the blades look more uniformly blurry. At

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Speaker 1: quicker shutter speeds, strange looking patterns or even discreet individual

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Speaker 1: blades will begin to appear. The second main factor to

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Speaker 1: consider is frame rate. We can start with a slow

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Speaker 1: motion analogy. Imagine you've got a sun dial. The shadow

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

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Speaker 1: let's say you take a picture of that sun dial

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Speaker 1: exactly once every minute. If you play those pictures in

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Speaker 1: order as a video, you'll see the shadows spinning rapidly

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Speaker 1: around the clock in the normal clockwise direction. If you

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Speaker 1: capture a frame exactly once every twenty four hours in

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Speaker 1: this video, the shadow will appear to stand still, and

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Speaker 1: if you take the picture once every twenty three hours

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Speaker 1: and fifty nine minutes, the dial in the video will

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Speaker 1: appear too slowly creep backward. The same principle is at

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Speaker 1: work in videos of helicopter blades, only complicated by greater

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Speaker 1: speed in both the rotation and the frame rate, and

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Speaker 1: by adding more blades. Frame rate is usually expressed in

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

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Speaker 1: four FPS video of a helicopter rotor that spins one

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Speaker 1: full rotation every second. In the video, each revolution will

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Speaker 1: thus be broken into twenty four frames. You will see

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Speaker 1: the blades rotating normally, just moving one of their full

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Speaker 1: rotation in each frame. But if the blades spin exactly

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Speaker 1: twenty four times each second and you're still shooting at

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Speaker 1: twenty four frames per second, each full revolution will be

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Speaker 1: represented by only one frame. The blades will arrive back

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Speaker 1: in their starting place each time the camera captures a frame,

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Speaker 1: so they'll look like they're standing still. But what if

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Speaker 1: you have blades that spin exactly twenty three times each

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Speaker 1: second and you're still shooting at twenty four fps. Each

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Speaker 1: frame will capture the blades having just made about nine

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Speaker 1: percent of a full rotation, the blades will always be

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Speaker 1: just a little bit behind where they are are in

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Speaker 1: the previous frame. Thus, in the final video, the blades

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

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Speaker 1: only way our cameras can trick us. Sometimes propellers and

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Speaker 1: helicopter blades caught on video can look s shaped or

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Speaker 1: even fragmented. This type of distortion is caused by the

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Speaker 1: method of pixel capture that's used in digital cameras. Most

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Speaker 1: digital video cameras today don't expose the whole frame all

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Speaker 1: at once, but instead sample a single line of pixels

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Speaker 1: at a time and update the frame line by line.

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Speaker 1: This is called a rolling shutter. With a rolling shutter,

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Speaker 1: any object moving extremely fast will be sampled in a

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Speaker 1: way that distorts its shape across the frame, leading to

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Speaker 1: spinning blades that look bent or broken, or appear to

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Speaker 1: be hovering separate from the aircraft. In fact, you can

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Speaker 1: try this out on your phone's camera by panning quickly

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Speaker 1: back and forth while you're taking a video. If your

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Speaker 1: camera uses a roll links shutter, the picture will be

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Speaker 1: distorted so that solid objects will appear to bend, like

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Speaker 1: rubber or Jello as you rotate the lens. Check out

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Speaker 1: the brainstuff channel on YouTube, and for more on this

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Speaker 1: and thousands of other topics, visit how stuff works dot com.