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During frame by frame watching of some video taken via Instagram app one noticed strange unwanted artifacts on the video caused by camera flash. Please, consider below several sample image frames taken from the video:

Sample image frames taken from the video

Here we clearly see three cases:

  1. No camera flash.
  2. Camera flash: expected camera flash effect.
  3. Camera flash: unexpected camera flash effect (artifacts: the left side is brighter than the right side).

The questions:

  1. Why this happens at all? How this is related to the physics of the filming process: camera flash speed light, image acquisition by the image sensor at 24..25 fps, video compression/decompression (guess the H.264 was used by Instagram app, video was taken at the end of 2018), etc.
  2. How to avoid such an artifacts next time?

Thanks in advance for the explanation.

Direct link to the full video: https://scontent-ort2-1.cdninstagram.com/v/t50.12441-16/45731449_1983300578418249_711802823897524155_n.mp4?_nc_ht=scontent-ort2-1.cdninstagram.com&oe=5D7924B5&oh=b2ddf05b200a10ababd105c8b23d7f23&dl=1

  • A simple solution is to use a camera with CCD sensor, which has global shutter. These are increasingly rare, you may find a used one that shoots 1080p but not higher resolution. – Rusty Core Sep 10 '19 at 18:54
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This is actually a collateral effect of the concept of rolling shutter. Most video devices do not expose all the frame area at once. They expose starting from one side and (up or down) and move from there. (The example has the camera rotated 90° so the effect is on the sides of the frame).

This can be either a mechanical element that moves or rotates or an electronic system.

As the flash of a camera is a really fast burst of light, it can sometimes be captured in one zone of the frame, by the time another part of the frame is streaming the data, the flash burst is over, so this zone of the image is transmitting the non-illuminated scene.

On still frame photography, this is the reason you have a minimum sync speed.

Frankly, it is really hard to avoid this situation in an environment such as the example posted, because you can not sync your camera to that flash.

On a controlled environment, like lighting in a movie, you could add the lighting in post-production, making one full frame white, and probably some other just a bit exposed.

Another way to control it is by using a slower burst of light. Rather than using a flash that lasts about a thousandth of a second (which disappears really fast), you use some system that flashes on tenths of a second. This way the burst of light has time to be captured by the shutter of the camera.

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    re the last point: you could also use a higher shutter speed if that doesn't produce other unwanted artefacts. – stib Sep 10 '19 at 6:25
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    Also, on the last point, don't forget about infinitely slow "flashes"; i.e., continuous light sources such as video lights or sunlight. – Jason Conrad Sep 13 '19 at 3:12
  • @stib, I actually want to simulate different shutter speeds to compare the effect, slower and faster than the 180° rule. – Rafael Sep 13 '19 at 4:40
  • Excellent answer, thanks! The @Rusty Core is right: the simple solution to reduce or eliminate unwanted image artifacts is to use a sensor with a global shutter. – pmor Sep 24 '19 at 14:55

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