Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain stuff,
Speaker 1: this is Christian Seger. Have you ever noticed that fast
Speaker 1: spinning objects like helicopter blades and airplane propellers sometimes look
Speaker 1: really weird on film and video. Sure, sometimes you can
Speaker 1: only see a circular blur, but sometimes they appear to
Speaker 1: be spinning very slowly. Sometimes they also look like they're
Speaker 1: spinning backward. In very rare cases, they can even look
Speaker 1: like they're holding still. So what is up with that? Well,
Speaker 1: when you watch a film or a video, nothing on
Speaker 1: screen is actually moving. Instead, you're seeing a succession of
Speaker 1: still images that come so rapidly that your eyes and
Speaker 1: brain interpret them as continuous motion. With this in mind,
Speaker 1: we need to look at two different facts about how
Speaker 1: those images are captured, shutter speed and frame rate. Shutter
Speaker 1: speed is a measure of how along the camera spends
Speaker 1: collecting light each time it takes a picture. The longer
Speaker 1: the film or digital pixel array spends gathering light, the
Speaker 1: more motion blur we see in the image. Let's say
Speaker 1: you want to shoot a video of yourself doing some
Speaker 1: rad throwing knife tricks. If you toss a knife at
Speaker 1: ten ms per second and film it at a shutter
Speaker 1: speed of one quarter of a second, the knife will
Speaker 1: travel two point five meters while the camera is exposing,
Speaker 1: each frame, coming out as a streaky blur in the
Speaker 1: final video. But if you shoot the same knife with
Speaker 1: a shutter speed of one one of a second, it
Speaker 1: will only travel one centimeter while each picture is taking.
Speaker 1: Beat it will only travel one centimeter while each picture
Speaker 1: is taken, meaning the knife will look less blurry in
Speaker 1: each frame. Now, the same applies to helicopter blades. Long
Speaker 1: exposures will make the blades look more uniformly blurry. At
Speaker 1: quicker shutter speeds, strange looking patterns or even discreet individual
Speaker 1: blades will begin to appear. The second main factor to
Speaker 1: consider is frame rate. We can start with a slow
Speaker 1: motion analogy. Imagine you've got a sun dial. The shadow
Speaker 1: on the dial makes one complete revolution every twenty four hours. Now,
Speaker 1: let's say you take a picture of that sun dial
Speaker 1: exactly once every minute. If you play those pictures in
Speaker 1: order as a video, you'll see the shadows spinning rapidly
Speaker 1: around the clock in the normal clockwise direction. If you
Speaker 1: capture a frame exactly once every twenty four hours in
Speaker 1: this video, the shadow will appear to stand still, and
Speaker 1: if you take the picture once every twenty three hours
Speaker 1: and fifty nine minutes, the dial in the video will
Speaker 1: appear too slowly creep backward. The same principle is at
Speaker 1: work in videos of helicopter blades, only complicated by greater
Speaker 1: speed in both the rotation and the frame rate, and
Speaker 1: by adding more blades. Frame rate is usually expressed in
Speaker 1: frames per second, or FPS. Imagine you're shooting a twenty
Speaker 1: four FPS video of a helicopter rotor that spins one
Speaker 1: full rotation every second. In the video, each revolution will
Speaker 1: thus be broken into twenty four frames. You will see
Speaker 1: the blades rotating normally, just moving one of their full
Speaker 1: rotation in each frame. But if the blades spin exactly
Speaker 1: twenty four times each second and you're still shooting at
Speaker 1: twenty four frames per second, each full revolution will be
Speaker 1: represented by only one frame. The blades will arrive back
Speaker 1: in their starting place each time the camera captures a frame,
Speaker 1: so they'll look like they're standing still. But what if
Speaker 1: you have blades that spin exactly twenty three times each
Speaker 1: second and you're still shooting at twenty four fps. Each
Speaker 1: frame will capture the blades having just made about nine
Speaker 1: percent of a full rotation, the blades will always be
Speaker 1: just a little bit behind where they are are in
Speaker 1: the previous frame. Thus, in the final video, the blades
Speaker 1: will look like they're spinning backwards. But that's not the
Speaker 1: only way our cameras can trick us. Sometimes propellers and
Speaker 1: helicopter blades caught on video can look s shaped or
Speaker 1: even fragmented. This type of distortion is caused by the
Speaker 1: method of pixel capture that's used in digital cameras. Most
Speaker 1: digital video cameras today don't expose the whole frame all
Speaker 1: at once, but instead sample a single line of pixels
Speaker 1: at a time and update the frame line by line.
Speaker 1: This is called a rolling shutter. With a rolling shutter,
Speaker 1: any object moving extremely fast will be sampled in a
Speaker 1: way that distorts its shape across the frame, leading to
Speaker 1: spinning blades that look bent or broken, or appear to
Speaker 1: be hovering separate from the aircraft. In fact, you can
Speaker 1: try this out on your phone's camera by panning quickly
Speaker 1: back and forth while you're taking a video. If your
Speaker 1: camera uses a roll links shutter, the picture will be
Speaker 1: distorted so that solid objects will appear to bend, like
Speaker 1: rubber or Jello as you rotate the lens. Check out
Speaker 1: the brainstuff channel on YouTube, and for more on this
Speaker 1: and thousands of other topics, visit how stuff works dot com.