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I was taking a picture of my class note with my mobile phone and after taking the snap, the photo appears to have some vertical darker bands.

the photo with vertical darker bands

The bands were moving horizontally from left to right when the camera lens were scanning the picture. When the shutter was clicked, the dark bands were captured. If observed carefully, there appears to be two dark bands.

What could have caused the dark bands?

39

The darker vertical bands? I would ascribe them to a synchronization between sensor capture and a slightly flickering lighting (one aspect of the "rolling shutter" problem). Is the exposure time of the picture roughly three periods(*) of your local current frequency (1/15-1/20 of a second)?

(*) At least three bands in the picture, beside the two obvious ones, there is one along the left border.

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    Another one starts at the right border. -> I think it's a rolling shutter with flickering light problem, too. – Horitsu Apr 15 at 12:06
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To expand on xenoids answer.

Most phone cameras use what is known as a "rolling shutter", the exposure starts and ends at slightly different times for different parts of the image. This makes the sensor cheaper because the end of the exposure can be defined by the readout process rather than needing extra electronics to capture the image at the end of the exposure.

This causes time-variations in the lighting level to be translated to spacial variations in the resulting image.

So if your light source varies in intensity at a speed a few times faster than the sensor readout time, you will get bars like this. How dark the bars are will depend on the exposure time the camera is using. Pointing your camera directly at the problem light will likely result in a shorter exposure time and hence stronger bars.

Many (but not all) flourescent and LED lights flicker at twice mains frequency, which tends to be in the same ballpark as sensor readout times.

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As the other answers note, this is due to a beat frequency between the readout frequency and the flicker frequency of the lighting.

You should check that the country/region of your phone is properly set. Also check the camera app for a power line rate in the settings.

A properly designed phone or video camera is able to compensate for lighting flicker by ensuring that integration times are an integer multiple of the power line rate, 50 or 60 Hz. This is called an anti-banding filter. Conversely, if the phone is set with the wrong power line rate, it will make the problem worse by forcing a 5/6 or 6/5 factor.

  • What are the chances that the camera app in the phone is "properly designed"? I mean, I have never seen power line rate setting in a camera app... – juhist Apr 16 at 6:57
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    @juhist maybe a very small chance for stock camera apps, but higher chance on "pro" and paid camera apps, like Camera FV-5 (I'm not affiliated, just a happy user after finding this feature on this app -- page 46 of the manual) – Andrew T. Apr 16 at 14:34
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    @juhist If you check the link in my answer, it shows that the stock Android camera system implements anti-banding. So all apps should by default. That's why I said to check system region, I believe most phones automatically select 50/60 Hz from that setting. The only reason why you normally need to manually set is for Japan which has mixed frequencies. – user71659 Apr 16 at 15:16
  • ensuring that integration times are an integer multiple of the power line rate Hmmm. this on the contrary guarantees that the bands will always be at the same place. Hard to imagine how that works for still photo, without uncomfortably increasing exposure time. – xenoid Apr 16 at 19:40
  • @xenoid No integer multiples is correct. Because when you're integrating full cycles, the phase no longer matters. This is why 60 Hz countries use 30/60 FPS video. (You can assume the two power line halves are identical, so it's actually 120 Hz) When exposure times are short, you ensure that you're integrating over the same portion of the flicker cycle. – user71659 Apr 16 at 21:04

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