Speaker 1: Welcome to brain Stuff production of I Heart Radio. Hey
Speaker 1: brain Stuff, I'm Lauren Vogelbaum, and this is another classic
Speaker 1: episode from our former host, Christian Sagar. For a look
Speaker 1: behind the scenes. Two of the writers who wrote these
Speaker 1: scripts back in the day were also on a future
Speaker 1: science and technology podcast called Forward Thinking at the time,
Speaker 1: me and Joe McCormick. So often conversations would have over
Speaker 1: there would influence the topics we picked for brain Stuff,
Speaker 1: including this one. Why can helicopter blades look so weird
Speaker 1: on video? Hey, brain Stuff, this is Christian Sager. Have
Speaker 1: you ever noticed that fast spinning objects like helicopter blades
Speaker 1: and airplane propellers sometimes look really weird on film and video. Sure,
Speaker 1: sometimes you can only see a circular blur, but sometimes
Speaker 1: they appear to be spinning very slowly. Sometimes they all
Speaker 1: so look like they're spinning backward. In very rare cases,
Speaker 1: they can even look like they're holding still. So what
Speaker 1: is up with that? Well, when you watch a film
Speaker 1: or a video, nothing on screen is actually moving. Instead,
Speaker 1: you're seeing a succession of still images that come so
Speaker 1: rapidly that your eyes and brain interpret them as continuous motion.
Speaker 1: With this in mind, we need to look at two
Speaker 1: different facts about how those images are captured, shutter speed
Speaker 1: and frame rate. Shutter speed is a measure of how
Speaker 1: long the camera spends collecting light each time it takes
Speaker 1: a picture. The longer the film or digital pixel array
Speaker 1: spends gathering light, the more motion blur we see in
Speaker 1: the image. Let's say you want to shoot a video
Speaker 1: of yourself doing some rad throwing knife tricks. If you
Speaker 1: toss a knife at ten ms per second and film
Speaker 1: it at a shutter speed of one quarter of a second,
Speaker 1: the knife will travel two point five meters while the
Speaker 1: camera is exposing each frame, coming out as a streaky
Speaker 1: blur in the final video. But if you shoot the
Speaker 1: same knife with a shutter speed of one one thousand
Speaker 1: of a second, it will only travel one centimeter while
Speaker 1: each picture is taken, meaning the knife will look less
Speaker 1: blurry in each frame. Now, the same applies to helicopter blades.
Speaker 1: Long exposures will make the blades look more uniformly blurry.
Speaker 1: At quicker shutter speeds, strange looking patterns, or even discreet
Speaker 1: individual blades will begin to appear. The second main factor
Speaker 1: to consider is frame rate. We can start with a
Speaker 1: slow motion analogy Imagine you've got a sun dial. The
Speaker 1: shadow on the dial makes one complete revolution every twenty
Speaker 1: four hours. Now, let's say you take a picture of
Speaker 1: that sun dial exactly once every minute. If you play
Speaker 1: those pictures in order as a video, you'll see the
Speaker 1: shadows spinning rapidly around the clock in the normal clockwise direction.
Speaker 1: If you capture a frame exactly one every twenty four
Speaker 1: hours in this video, the shadow will appear to stand still.
Speaker 1: And if you take the picture once every twenty three
Speaker 1: hours and fifty nine minutes, the dial in the video
Speaker 1: will appear too slowly creep backward. The same principle is
Speaker 1: at work in videos of helicopter blades, only complicated by
Speaker 1: greater speed in both the rotation and the frame rate,
Speaker 1: and by adding more blades. Frame rate is usually expressed
Speaker 1: in frames per second, or FPS. Imagine you're shooting a
Speaker 1: twenty four FPS video of a helicopter rotor that spins
Speaker 1: one full rotation every second. In the video, each revolution
Speaker 1: will thus be broken into twenty four frames. You will
Speaker 1: see the blades rotating normally, just moving one of their
Speaker 1: full rotation in each frame. But if the blades spin
Speaker 1: exactly twenty four times each second, and you're still shooting
Speaker 1: at twenty four frames per second, each full revolution will
Speaker 1: be represented by only one frame. The blade will arrive
Speaker 1: back in their starting place each time the camera captures
Speaker 1: of frames, so they'll look like they're standing still. But
Speaker 1: what if you have blades that spin exactly twenty three
Speaker 1: times each second and you're still shooting at fps. Each
Speaker 1: frame will capture the blades having just made about of
Speaker 1: a full rotation, The blades will always be just a
Speaker 1: little bit behind where they are in the previous frame. Thus,
Speaker 1: in the final video, the blades will look like they're
Speaker 1: spinning backwards. But that's not the only way our cameras
Speaker 1: can trick us. Sometimes propellers and helicopter blades caught on
Speaker 1: video can look s shaped or even fragmented. This type
Speaker 1: of distortion is caused by the method of pixel capture
Speaker 1: that's used in digital cameras. Most digital video cameras today
Speaker 1: don't expose the whole frame all at once, but instead
Speaker 1: sample a single line of pixels at a time and
Speaker 1: update the frame line by line. This is called a
Speaker 1: rolling shutter. With a rolling shutter, any object moving extremely
Speaker 1: fast will be sampled in a way that distorts its
Speaker 1: shape across the frame, leading to spinning blades that look
Speaker 1: bent or broken, or appear to be hovering separate from
Speaker 1: the aircraft. In fact, you can try this out on
Speaker 1: your phone's camera by panning quickly back and forth while
Speaker 1: you're taking a video. If your camera uses a rolling shutter,
Speaker 1: the picture will be distorted so that solid objects will
Speaker 1: appear to bend, like rubber or jello as you rotate
Speaker 1: the lens. Today's episode was written by Joe McCormick and
Speaker 1: produced by Tyler Playing. To hear more from Joe, check
Speaker 1: out his current science show, Stuff to Blow Your Mind,
Speaker 1: and of course, for more on this and lots of
Speaker 1: other topics, is it how stuffworks dot com. Brain Stuff
Speaker 1: is production of by Heart Radio. For more podcasts, my
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