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 be spinning very
slowly.
Sometimes they look like they’re spinning
backward.
In very rare cases, they can even look like
they’re holding still.
What’s up with that?
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 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 long the
camera spends collecting light each time it
takes a picture.
The longer the film or digital pixel array
spends gathering light, the more motion-blur
we see in the image.
Let’s say you want to shoot video of yourself
doing some rad throwing knife tricks.
If you toss a knife at 10 meters per second
and film it at a shutter speed of one-quarter
of a second, the knife will travel 2.5 meters
while the camera is exposing each frame, coming
out as a streaky blur in the final video.
But if you shoot that same knife with a shutter
speed of one one-thousandth of a second, it
will only travel 1 centimeter while each picture
is taken.
Meaning the knife will look less blurry in
each frame.
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 discrete individual blades
will begin to appear.
The second main factor to consider is frame
rate.
Frame rate is usually expressed in frames-per-second,
or "fps."
Imagine you're shooting a 24 frames-per-second
video of a helicopter rotor that spins one
full rotation every second.
In the video, each revolution will thus be
broken into 24 frames.
You'll see the blades rotating normally - just
moving 1/24th of their full rotation in each
frame.
But if the blades spin exactly 24 times each
second, and you’re still shooting at 24
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 you have blades that spin exactly
23 times each second, and you’re still shooting
at 24 FPS?
Each frame will capture the blades having
just made about 96 percent of a full rotation.
The blades will always be a little bit behind
where they were in the previous frame.
Thus, in the final video, the blades will
look like they’re spinning backwards.
So this video in particular is probably a
result a very fast shutter speed combined
with a frame rate synchronized almost perfectly
to the rate of the blades’ rotation, or
some whole multiple thereof.
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 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 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 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 rolling shutter, the
picture will be distorted so that solid objects
will appear to bend like rubber or jello as
you rotate the lens.
Try it out and let us know how it goes in
the comments!
And for more on topics related to film and
photography, check out HowStuffWorks.com.
