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Why Can Helicopter Blades Look Weird on Video?



Have you ever noticed that fast spinning objects like helicopter blades in 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 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 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 cameras 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 WANNA shoot a video of yourself. Doing some radd throwing knife tricks if you toss a knife at ten meters 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 final video. But if you shoot the same knife with a shutter speed of one one thousandth of a second. It will only travel one centimeter while each picture is taken meaning. The knife will look less blurry in 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 Sundial. The shadow on the dial mix one complete revolution every twenty four hours now. Let's say you take a picture of that Sundial once every minute. If you play those pictures in order as a video you'll see the shadow spinning rapidly around the clock in the normal clockwise direction. If you capture a frame exactly once every twenty four hours in 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 to 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 by adding more blades frame rate is usually expressed in frames per second or F-. Ps Imagine you're shooting twenty four F. Ps 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 twenty fourth of their full 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 you have blades that spin exactly twenty three times each second in? You're still shooting at twenty four F. Ps each frame will capture the blades. Having just made about ninety six percent of a full rotation the blades will always be just a little bit behind where they are in the previous frame thus in the final video the blades 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 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 Panning. Quickly back and forth while you're taking video if your camera uses a rolling shutter the picture will be distorted so that solid objects will appear to bend like rubber or

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