Here’s Why Helicopter Blades Can Look Strange On Video


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.

100 Replies to “Here’s Why Helicopter Blades Can Look Strange On Video”

  1. The helicopter rotors in your animations rotate in the wrong direction. The tapering part (the blade root cutout) is at the trailing edge of the blade; thus, as depicted, the blades should be rotating counterclockwise. Other than that, it’s a solid, if somewhat longwinded, explanation.

  2. Really, some young girl decided to make this video because these is just sooooo much confusion out there that need correction. Misinformation at its best.

  3. How to fuck up a informative video:

    When she is breaking down FPS, and says the blades are rotating "normally". They aren't. The trailing edge of the blade is leading in the rotation, which is backwards. And visa versa for when she says it's rotating "backwards".

  4. And just how close would the frequency or frame rate have to match? You have better odds at the lottery me thinks. Oh we have one that Can SEE….

  5. I thought in video, shutter speed is represented as an angle. The fraction is only for still images (yes I know video is made up of still images)

  6. hey,i am getting confused more. is it like that higher the frame rate is directly proportional to shutter speed? thank you.

  7. Global shutters for the win. Smartphones should really stop using sensors that use rollong shutters. I abhor distorted images, especially those of rotors.

  8. So that spinning backwards thing is because of the lower frame rate compared to the frequency of rotation of the helicopter blades, or something like that right?
    That thing happens with our eyes too. For example when you are on the road and try to fix your eyes on a car's wheel next beside you. The wheel, for some moments starts to spin backwards as well.
    Sooo, does that mean our eyes have a frame rate? 🤔 Hahaha

  9. When we say frames per second we mean pictures per second. So if you draw a lot of pictures you can actually make a film. But you need millions

  10. Okay, thanks. My friend and I were at a World War II airshow and we were taking video of planes taking off, but their rotors appeared to move really slow or backwards like you said.

  11. This isn't correct as far as video capture. The shutter stays open and captures light on the image sensor. It's the image sensor that refreshes however many times per second the video is recorded. In this she says the shutter is the one that is refreshing X times per second. Also why rolling shutter is named this I am uncertain because it actually doesn't have to do with the shutter. The 2 types of shutter are leaf and focal plane. This "rolling shutter" (which should be called progressive scanning instead) is not present in many cameras. CMOS sensors capture the entire frame as mentioned before by utilizing the entire image sensor at once. Therefore they do not suffer from this distortion. Also almost all cameras use a focal place shutter. It's essentially a curtain that moves very fast across the focal plane while another curtain follows behind it.

  12. Try looking at your hand shaking it as fast as you can and blinking your eyes constantly and rapidly !!!!

    You'll definitely understand now …

  13. Great video, but it's probably more accurate to say that the blades are moving at a much higher number of times per second that are evenly divisible by the frame rate. So, if the frame rate is 30, the RPM could be something like 300 or 330 RPM. … or, even a number divisible by 1/6 since there are 6 blades on the heli, and the video could be "swapping out" blades to still give us that illusion. Just a guess.

  14. Didn't know about bend object due to rolling shutter but is there any advantage of using rolling sutter??

  15. This is a very showing way of manipulating one's perception of reality and Viktor Schauberger wrote of this as he handed out answers instead of flawed theories and ignorance being handed out by people to maintain a lower level of thinking for the unworthy.

  16. Not bad, pretty close. Actually it revolves around the shutter speed per second and the frames per second. Both must come into play for the blades to appear motionless. Frames and shutter speeds dont always match in cameras. Say the frames per sec in ur camera are 32 and shutter speed is 33 and the helo blades are rotating at 32 rotations per sec. The blades will appear slightly blurry and look as if their in supper slow motion rotating clockwise. Now if the blades and camera frames were at 33 per sec and ur shutter at 32 the helo blades would look. Slightly blurry in supper slow motion going counterclockwise.. Hope this helped. PS- regular helo & not a gov secret flight..

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