Why aren’t objects bent in videos where the frequency of said objects is the same as the camera shutter speed?


The title won’t make sense if I don’t explain as I barely understand the basics. I have recently learned about the rolling shutter effect from this video:

I have seen several posts here on reddit illustrating what happens when the camera shutter is the same as for example the blades of a helicopter. I get that the blades appear to not move but why I don’t get is why the blades aren’t frozen and also bent as seen in the rolling shutter effect.

Example of helicopter blades for example:

In: Technology

The rolling shutter effect is only visible in certain conditions. Some more expensive cameras have a global shutter option instead of a rolling shutter. And whith a global shutter you do not get the rolling shutter effect. Another way to get rid of the rolling shutter effect is to increase the shutter speed so there is less movement in the frame as the rolling shutter is moving across it. So it is quite likely that they have used camera settings that does not show the rolling shutter effect when filming the helicopter blades. Either they are using equipment with a global shutter or they are filming it on a bright day with a faster shutter so the effect is not so visible.

There are at least ways of capturing an image by using a rolling shutter or a global shutter.

In a film camera and even in a DSLR today you have a mechanical rolling shutter that mover in front of the film/ sensor looks at them [in this video clip](https://youtu.be/CmjeCchGRQo?t=138). So the top of the senor is exposed before the bottom. If you have a short shutter speed the distance between the two shutters is small and it looks [like this part of the video](https://youtu.be/CmjeCchGRQo?t=251). So you capture the image through a narrow moving horizontal window.
If you capture an image of a fast-moving object that way it will be distorted because it will move and be in a different position for different parts of the sensor.

Even in camera no physical shutter the common CMOS sensor you read it outline for the line starting at the top and the lowe part continued to gather light so you have an electronic rolling shutter with the exact same effect.

In a more expensive and more sensitive CCD sensor, you have a global shutter and capture the whole image at the same time. They do not have the artifact

There’s definitely some video artifacts similar to this. Like when helicopter blades, fans, and car wheels can appear to be turning backwards at certain speeds.

Former game developer here,

This totally happens. When I was in game development, 32fps was the ideal render speed, now days it’s 60fps.

So imagine we have a rotating object in the game. Every 1/60 of a second the object will have rotated some number of degrees and we render that single frame. You can imagine the object may rotate just less than 360 degrees every frame. What would that look like? A point of reference is behind where it was previously, when it should be ahead! You’d see it in like a rotating wheel of a car, for example.

If you have a robust render engine, it would actually catch this sort of artifact and mitigate it with heuristics. Ideally you would limit rotation so that the object always rotates enough to look like it’s advanced, but not so much that it looks like it rotated backward.

That video title “camera shutter speed and frame rate match helicopter`s rotor” is not entirely correct. The rotor’s rotational speed is almost an exact multiple of the frame rate, so saying that those match is pretty much true. However, there’s no “matching” of shutter speed. The shutter speed is simply extremely fast in that video, which is why the blades are individually visible. If that video used a more normal shutter speed, those blades would just be a blurry disk.

Syncing of shutter speeds is more relevant to flickering and artificial lighting in videos.

The camera used in that helicopter video has a global shutter, which is why you don’t see any warping of the rotor blades.