WEBVTT - BrainStuff Classics: Why Can Helicopter Blades Look Weird on Video?

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<v Speaker 1>Welcome to brain Stuff production of I Heart Radio. Hey

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<v Speaker 1>brain Stuff, I'm Lauren Vogelbaum, and this is another classic

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<v Speaker 1>episode from our former host, Christian Sagar. For a look

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<v Speaker 1>behind the scenes. Two of the writers who wrote these

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<v Speaker 1>scripts back in the day were also on a future

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<v Speaker 1>science and technology podcast called Forward Thinking at the time,

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<v Speaker 1>me and Joe McCormick. So often conversations would have over

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<v Speaker 1>there would influence the topics we picked for brain Stuff,

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<v Speaker 1>including this one. Why can helicopter blades look so weird

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<v Speaker 1>on video? Hey, brain Stuff, this is Christian Sager. Have

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<v Speaker 1>you ever noticed that fast spinning objects like helicopter blades

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<v Speaker 1>and airplane propellers sometimes look really weird on film and video. Sure,

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<v Speaker 1>sometimes you can only see a circular blur, but sometimes

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<v Speaker 1>they appear to be spinning very slowly. Sometimes they all

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<v Speaker 1>so look like they're spinning backward. In very rare cases,

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<v Speaker 1>they can even look like they're holding still. So what

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<v Speaker 1>is up with that? Well, when you watch a film

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<v Speaker 1>or a video, nothing on screen is actually moving. Instead,

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<v Speaker 1>you're seeing a succession of still images that come so

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<v Speaker 1>rapidly that your eyes and brain interpret them as continuous motion.

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<v Speaker 1>With this in mind, we need to look at two

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<v Speaker 1>different facts about how those images are captured, shutter speed

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<v Speaker 1>and frame rate. Shutter speed is a measure of how

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<v Speaker 1>long the camera spends collecting light each time it takes

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<v Speaker 1>a picture. The longer the film or digital pixel array

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<v Speaker 1>spends gathering light, the more motion blur we see in

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<v Speaker 1>the image. Let's say you want to shoot a video

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<v Speaker 1>of yourself doing some rad throwing knife tricks. If you

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<v Speaker 1>toss a knife at ten ms per second and film

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

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<v Speaker 1>the knife will travel two point five meters while the

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<v Speaker 1>camera is exposing each frame, coming out as a streaky

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<v Speaker 1>blur in the final video. But if you shoot the

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<v Speaker 1>same knife with a shutter speed of one one thousand

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<v Speaker 1>of a second, it will only travel one centimeter while

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<v Speaker 1>each picture is taken, meaning the knife will look less

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

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

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

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

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

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

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<v Speaker 1>shadow on the dial makes one complete revolution every twenty

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<v Speaker 1>four hours. Now, let's say you take a picture of

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<v Speaker 1>that sun dial exactly once every minute. If you play

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<v Speaker 1>those pictures in order as a video, you'll see the

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<v Speaker 1>shadows spinning rapidly around the clock in the normal clockwise direction.

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<v Speaker 1>If you capture a frame exactly one every twenty four

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<v Speaker 1>hours in this video, the shadow will appear to stand still.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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<v Speaker 1>be represented by only one frame. The blade will arrive

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<v Speaker 1>back in their starting place each time the camera captures

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<v Speaker 1>of frames, so they'll look like they're standing still. But

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<v Speaker 1>what if you have blades that spin exactly twenty three

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

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

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<v Speaker 1>a full rotation, The blades will always be just a

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<v Speaker 1>little bit behind where they are in the previous frame. Thus,

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<v Speaker 1>in the final video, the blades will look like they're

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<v Speaker 1>spinning backwards. But that's not the only way our cameras

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<v Speaker 1>can trick us. Sometimes propellers and helicopter blades caught on

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<v Speaker 1>video can look s shaped or even fragmented. This type

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<v Speaker 1>of distortion is caused by the method of pixel capture

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<v Speaker 1>that's used in digital cameras. Most digital video cameras today

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<v Speaker 1>don't expose the whole frame all at once, but instead

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<v Speaker 1>sample a single line of pixels at a time and

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<v Speaker 1>update the frame line by line. This is called a

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<v Speaker 1>rolling shutter. With a rolling shutter, any object moving extremely

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<v Speaker 1>fast will be sampled in a way that distorts its

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<v Speaker 1>shape across the frame, leading to spinning blades that look

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<v Speaker 1>bent or broken, or appear to be hovering separate from

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<v Speaker 1>the aircraft. In fact, you can try this out on

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<v Speaker 1>your phone's camera by panning quickly back and forth while

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<v Speaker 1>you're taking a video. If your camera uses a rolling shutter,

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<v Speaker 1>the picture will be distorted so that solid objects will

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<v Speaker 1>appear to bend, like rubber or jello as you rotate

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<v Speaker 1>the lens. Today's episode was written by Joe McCormick and

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<v Speaker 1>produced by Tyler Playing. To hear more from Joe, check

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<v Speaker 1>out his current science show, Stuff to Blow Your Mind,

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<v Speaker 1>and of course, for more on this and lots of

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<v Speaker 1>other topics, is it how stuffworks dot com. Brain Stuff

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<v Speaker 1>is production of by Heart Radio. For more podcasts, my

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