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enter image description here

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I was in class and took a picture of the slides with my phone (iPhone 13 if that helps) but the first one (I zoomed in a little on that one) turned out as shown while the next one was fine. The two figures in front are probably my classmates sitting down but nobody was standing in the back so why are there blurry spots there and why did this happen when I’ve never had a problem with my pic before? I took more later and they were fine too. Thanks!

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This phenomenon entirely depends on how backlight of the LCD works. The brightness of LEDs is controlled with very frequent pulses (Pulse Width Modulation) which are mostly unnoticeable to humans. PWM works at fixed frequency and the duration of the pulse is what defines the brightness. Low PWM frequency is cheaper in production.

If your shutter speed is close to that pulse duration you will either get varying object brightness or even a completely dark screen.

The light being slightly visible on the edges is consequence of shutter acting gradually across the image (rolling shutter), so top and bottom of the screen were photographed at different moments.

You may observe similar phenomenon in other cases where LED lighting or gas discharge lamps are used.

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    \$\begingroup\$ This also is a problem when taking a picture with LED-based traffic lights in frame. On videos this becomes especially jarring, because the lights appear to flicker badly on the recorded video, completely misrepresenting the actual performance of the traffic light. \$\endgroup\$
    – Ruslan
    Commented Sep 15, 2023 at 19:34
  • \$\begingroup\$ @Ruslan: video should be shot at 1/50s, more than enough to oversample the PWM strobing. \$\endgroup\$
    – dandavis
    Commented Sep 15, 2023 at 21:50
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    \$\begingroup\$ @dandavis that's the frame rate, but not necessarily the exposure of each frame. In bright daytime conditions the exposure will likely be shorter. \$\endgroup\$
    – Ruslan
    Commented Sep 15, 2023 at 21:51
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    \$\begingroup\$ @Ruslan well, I was speaking for a 24fps frame rate. If you shoot under 180 deg shutter angle (aka 2 X FPS), you get no motion blur and the video looks terrible and amateurish, like it came from a phone or camcorder; uncinematic. \$\endgroup\$
    – dandavis
    Commented Sep 15, 2023 at 21:55
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    \$\begingroup\$ @dandavis OK, I was talking about video shot by a phone (and also by dashboard cameras), where too often do I notice the flicker of traffic lights. \$\endgroup\$
    – Ruslan
    Commented Sep 15, 2023 at 21:58

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