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This image was taken during a thunderstorm and I am trying to find an explanation for it. My guess is that it is a digital artifact because the digital camera's mechanisms for reading pixels data from CCD sensor and the delay associated with reading and storing each frame. So, I am wondering wether my guess is correctenter image description here

What is the scientific explanation of this lightning image?

Here is a link to a video clip that contains the image https://mobile.twitter.com/307Mod/status/1045366144838717440/video/1

  • Is this a still frame extracted from a video? – mattdm Oct 3 '18 at 18:47
  • @mattdm Yes, According to the owner. – Mohammad Al-Turkistany Oct 3 '18 at 18:48
  • Although some similar things affect photography, given that you're specifically interested in video this is likely better at video.stackexchange.com – mattdm Oct 3 '18 at 18:50
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    actually it is not a video frame. the linked video shows a smartphone app reviewing a series of photos taken in the burst mode. so even if the picture is extracted from a video, it is still a photo, taken in a photo capture mode. of course, smartphone means electronic shutter, so it's similar to what "serious" cameras do in video mode. – szulat Oct 3 '18 at 20:04
  • @szulat Yes, I think you are right. The resolution is much better than a video frame. – Mohammad Al-Turkistany Oct 3 '18 at 20:07
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Cameras design utilizes several different shutter designs. The DSLR (Digital Single Lens Reflex) typically use a focal plane shutter design. Such a shutter is favored when the camera supports interchangeable lenses. This is because the shutter resides at the rear of camera body; it hovers just above the surface of the image sensor. This design features a curtain with a slit. When the shutter button is actuated, the curtain with its slit opening travels across the image sensor. The shutter speed is the clock time it takes to travel the width of the slit. In other words, if the shutter speed is set to 1/125 of second, the curtain moves 1 slit width in 1/125 of a second. If the shutter speed is set faster, the width of the slit is set narrower, if the shutter speed is set slower, the slit width is set wider.

In my opinion, this image was taken during a lightning flash. The flash of light produced by lightening can be quite short (milliseconds). I think the shutter speed was set fast and as fortune happened, the lighting strike caught the lighting flash as the shutter was in motion. The part of sensor uncovered, recorded an image that was well exposed. Part of the slit travel exposed the sensor without the benefit of the lightning strike, this region of the chip is under-exposed. We see this happen all the time when the photographer is using an electronic flash. If the blitz of the flash and the shutter slit motion are not synchronized, we get this effect. Let me add, it’s not easy, perhaps impossible, to synchronized a shutter with a lightening flash.

  • Nice catch, Alan. – Stan Oct 3 '18 at 20:33
  • Could you explain why the flash causes the sky to be exposed differently? – Yogu Oct 3 '18 at 22:16
  • The slomo guys have a video that shows the slit geometry very clearly here: youtube.com/watch?v=CmjeCchGRQo -- in particular, fast forward to about the 3 minute mark. – Eric Lippert Oct 3 '18 at 22:34
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    @ Yogu - One half of the image is exposed under ambient light conditions. The other half is exposed under the bright bluish lighting flash. – Alan Marcus Oct 3 '18 at 23:07
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    @Yogu the sky is full of clouds which are illuminated by the lightning just as well as the ground is. I would say that since there are no hard shadows on the ground and buildings, the lightning probably went off inside the cloud, making it a mother of all lightboxes. – IMil Oct 4 '18 at 1:01
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Looks like the rolling shutter effect, which is more pronounced in (but not limited to) the video mode.

Like you suspected, the effect is caused by different parts of the image being captured at slightly different times. Electronic shutters (used in the video mode) are currently about 10 times slower than mechanical shutter, which means more chance for dramatic changes in the frame during the exposure.

Google search for "lightning rolling shutter" reveales interesting examples.

  • It takes approximately 2-4 milliseconds for a typical mechanical shutter curtain to transit a APS-C or FF sensor. Are you saying the fastest electronic shutters require 20-40 milliseconds to do a single readout? That's 1/50 to 1/25 seconds. How would 60 fps or 120 fps be possible at such slow speeds? – Michael C Oct 3 '18 at 20:30
  • they are simply not possible ;-) at full resolution. even some new cameras from top manufacturers don't support 4K video recording! but of course things are getting better each year... – szulat Oct 3 '18 at 20:41
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    oh, and i mean typical electronic shutters, not the fastest ones. sony A9 seems to be well ahead of the competition here (but still slower than mechanical). and of course it would be nice to see a global shutter consumer camera some day... – szulat Oct 3 '18 at 20:52
  • Yet there are plenty of cameras that do offer 60 fps, 120fps, or even faster with no cycling of a mechanical shutter between frames. – Michael C Oct 4 '18 at 4:33
  • @MichaelClark Possible with you have several reading channels. In video it is not a problem to start sampling the next frame well before you are done with the current one. – xenoid Oct 4 '18 at 5:54

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