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I was trying to take pictures (Samsung Note 20, both front and back camera) of the beautiful lights in a cafe in Seoul (Humbolt Cafe, 325-17 Seongsu 2(i)-ga 1(il)-dong, Seongdong-gu, Seoul).

But for some reason, the lines are appearing in front of it. When there is an object in front of the lights, the lines are gone.

enter image description hereenter image description here

What is the reason for this?

The first image, with the black lines, is taken with the following settings:

  • Aperture: F2.2
  • Shutter speed: 1/588
  • Focal length: 3.30 mm
  • ISO: 50
  • White balance: Auto
  • No flash

The second image, with the hand in front of the lamps, has all the same settings as the first except for the shutter speed:

  • Shutter speed: 1/130

I took an extra photo to see if it is because of dirt on my lens, but this photo appears to be fine.

enter image description here

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  • @Tetsujin : Thank you for the suggestion but no. I was in a very well-lit cafe. With a lot of lights (along with the ones I was trying to photograph). Also, I am using a phone to take the picture. We took a lot of pictures and all are without any lines if these lights are not there. – Devashish Das May 15 at 14:45
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    @Tetsujin That would require both a strong rolling shutter effect and it would require all relevant light sources in the room to be connected to the same pulse-width modulation dimming circuit, with a light source that reacts almost immediately to the modulation which would be something like LED on DC. Are they doing things like that? – user98068 May 15 at 14:52
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    Why are the pictures not the same size? Has the first one been cropped? I assume both are with the "Selfie" camera? – xenoid May 17 at 8:29
  • @xenoid No idea. It is not cropped by any way. It directly uploaded as taken. Yes both are from the front camera. – Devashish Das May 18 at 17:27
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The lights are either fluorescent or LED. This means that they turn on and off 50/60 times a second depending on the country.

The camera scans across the sensor at a relatively slow (constant) rate, but the photosites sample the incoming light for longer or shorter times depending on the scene (this time is equivalent to shutter speed). This is an exposure method called "rolling shutter" and is common with electronic shutters.

The first image with the dark bars uses a fast 'shutter speed' and shows the light/dark cycle of the light pretty clearly. The second image has some of the light blocked by the hand, and so uses a slower 'shutter speed' which is near one light/dark cycle multiple long and mostly evens out the lighting.

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    Because of rectification of the supply voltage, the lights turn on and off 120 times a second (South Korea runs on 60 Hz according to Wikipedia) – scottbb May 15 at 16:22
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    @scottbb I thought of that, but I didn't want to complicate the answer, and I wasn't sure it was universally true. A rectifier is another component and probably not necessary for visually non-flickering light (60Hz vs 120Hz). – BobT May 15 at 19:55
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    Modern fluorescent lamps with electronic ballasts can flicker at 10 kHz or faster. – Eric S May 15 at 22:31
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    @DevashishDas many modern cameras can compensate for this, but often you have to give them the frequency. India, where your profile says you are from, runs at 50 Hz, but South Korea runs at 60 Hz. It is possible that you have set to compensate the wrong frequency, and then you get the banding. – Davidmh May 16 at 17:30
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    @xenoid Sure it does. The readout rate of the imaging chip is the same in both. The EXPOSURE time (time the photosites are gathering photons) are different. – BobT May 17 at 13:35
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The second image also has bars, a bit less visible. The first image has 3, the second has 4, so are the exposure times in the same ratio?

It could be that the light you are blocking with your hand is blinking while the others are not. The exposure time would tell. South Korea is a 60Hz country so if the pictures are at 1/20s (3 bands) or 1/15s (4 bands) then you are blocking one of their last fluorescent bulbs and the rest of the lighting uses LED.

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    It's difficult to find the scan rate for rolling shutters (rate at which the camera scans across the photosites) but one of the few numbers I found was 20 Hz, which would correspond to your 1/20 of a second. The first image looks a little cropped compared to the second so that may account for the extra partial bars along the edges of the second. – BobT May 16 at 14:31
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    Compare the difference in the brightness of the ceiling behind the lights. The difference in Tv is far greater than 1/15 and 1/20. Please remember most phone cameras can not change aperture, so ISO and Tv are the only way to alter exposure. The first image is taken at a much shorter Tv than the second because the camera metered the scene as much brighter when the light was not obstructed, thus the difference in brightness of the ceiling. – Michael C May 16 at 19:57
  • LED lights get pulse width modulation at significantly higher than mains frequency or they'd be too flickery given their fast response time compared to filament bulbs. I think that may hold even for "dimmable" LED lights: I'd guess they measure the duty cycle of the 100Hz/120Hz partial phase signal they are fed with and translate it to the higher frequency PWM they push out. Rolling shutter may be slow, but not that slow. – user98068 May 17 at 23:48
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You are probably photographing through some kind of grid (like stabilising wires in a glass pane). In the second image, the hand blocks a significant amount of light, leading to a wider aperture chosen by the camera. A wider aperture in turn causes out-of-focus objects (like the rather close wires) to become blurred more strongly, here to the degree where the camera manages to "look around" the wires mostly due to the apparent aperture opening (the "entrance pupil") being wider than the wires.

The light in the first photograph is quite brighter and the camera reduces the light by closing down the aperture (it is apparent that the walls are quite darker). This smaller aperture opening can no longer "look around" the wires and they then manage to block sight completely at least in their middle.

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    pretty sure the OP would be aware if they were shooting through reinforced glass… or prison bars. – Tetsujin May 15 at 15:09

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