Why Do Helicopter Blades Look Weird On Video?

Helicopter blades and other fast spinning objects often produce strange effects on camera. Christian explains why.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works. Hey, brain stuff,
this is Christian Seger. Have you ever noticed that fast
spinning objects like helicopter blades and airplane propellers sometimes look
really weird on film and video. Sure, sometimes you can
only see a circular blur, but sometimes they appear to

(00:24):
be spinning very slowly. Sometimes they also look like they're
spinning backward. In very rare cases, they can even look
like they're holding still. So what is up with that? Well,
when you watch a film or a video, nothing on
screen is actually moving. Instead, you're seeing a succession of
still images that come so rapidly that your eyes and

(00:47):
brain interpret them as continuous motion. With this in mind,
we need to look at two different facts about how
those images are captured, shutter speed and frame rate. Shutter
speed is a measure of how along the camera spends
collecting light each time it takes a picture. The longer
the film or digital pixel array spends gathering light, the

(01:08):
more motion blur we see in the image. Let's say
you want to shoot a video of yourself doing some
rad throwing knife tricks. If you toss a knife at
ten ms per second and film it at a shutter
speed of one quarter of a second, the knife will
travel two point five meters while the camera is exposing,
each frame, coming out as a streaky blur in the

(01:31):
final video. But if you shoot the same knife with
a shutter speed of one one of a second, it
will only travel one centimeter while each picture is taking.
Beat it will only travel one centimeter while each picture
is taken, meaning the knife will look less blurry in

(01:52):
each frame. Now, the same applies to helicopter blades. Long
exposures will make the blades look more uniformly blurry. At
quicker shutter speeds, strange looking patterns or even discreet individual
blades will begin to appear. The second main factor to
consider is frame rate. We can start with a slow
motion analogy. Imagine you've got a sun dial. The shadow

(02:15):
on the dial makes one complete revolution every twenty four hours. Now,
let's say you take a picture of that sun dial
exactly once every minute. If you play those pictures in
order as a video, you'll see the shadows spinning rapidly
around the clock in the normal clockwise direction. If you
capture a frame exactly once every twenty four hours in

(02:35):
this video, the shadow will appear to stand still, and
if you take the picture once every twenty three hours
and fifty nine minutes, the dial in the video will
appear too slowly creep backward. The same principle is at
work in videos of helicopter blades, only complicated by greater
speed in both the rotation and the frame rate, and

(02:58):
by adding more blades. Frame rate is usually expressed in
frames per second, or FPS. Imagine you're shooting a twenty
four FPS video of a helicopter rotor that spins one
full rotation every second. In the video, each revolution will
thus be broken into twenty four frames. You will see
the blades rotating normally, just moving one of their full

(03:19):
rotation in each frame. But if the blades spin exactly
twenty four times each second and you're still shooting at
twenty four frames per second, each full revolution will be
represented by only one frame. The blades will arrive back
in their starting place each time the camera captures a frame,
so they'll look like they're standing still. But what if

(03:43):
you have blades that spin exactly twenty three times each
second and you're still shooting at twenty four fps. Each
frame will capture the blades having just made about nine
percent of a full rotation, the blades will always be
just a little bit behind where they are are in
the previous frame. Thus, in the final video, the blades

(04:03):
will look like they're spinning backwards. But that's not the
only way our cameras can trick us. Sometimes propellers and
helicopter blades caught on video can look s shaped or
even fragmented. This type of distortion is caused by the
method of pixel capture that's used in digital cameras. Most
digital video cameras today don't expose the whole frame all

(04:27):
at once, but instead sample a single line of pixels
at a time and update the frame line by line.
This is called a rolling shutter. With a rolling shutter,
any object moving extremely fast will be sampled in a
way that distorts its shape across the frame, leading to
spinning blades that look bent or broken, or appear to

(04:49):
be hovering separate from the aircraft. In fact, you can
try this out on your phone's camera by panning quickly
back and forth while you're taking a video. If your
camera uses a roll links shutter, the picture will be
distorted so that solid objects will appear to bend, like
rubber or Jello as you rotate the lens. Check out

(05:13):
the brainstuff channel on YouTube, and for more on this
and thousands of other topics, visit how stuff works dot com.