Why do those moving black bars appear when trying to take a photo of an older tv or computer screen? And not on modern ones?
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This probably has to do with how CRT’s (tube televisions) worked in comparison with newer digital technology.
So, back in the day, there was no such thing as a pixel-based display. The computer technology to control thousands of pixels individually didn’t exist. What tv’s did instead was use a single beam of electrons, and sweep it very quickly over the surface of a phosphorescent screen to create an image. Basically the beam would start in the upper-left corner, and create a level of brightness on that spot, and sweep across to the upper-right-hand corner, changing the brightness as it went, to create a whole line of the image. Then it goes to the next line and creates that one and so on. This sounds like it shouldn’t actually work, but it does, because 1. this is happening incredibly fast (covering the whole screen 30 times per second), and 2. our eyes are actually pretty bad at discerning small changes in images, such that as long as the beam moves fast enough we will perceive the whole image.
What’s happening when you try to take a photo of a CRT is that the shutter of the camera is freeze-framing a moment that your eyes can’t actually perceive. The beam is near the top or the bottom of the screen in that precise moment, so some of the parts of the screen are not actually lit up by the beam.
This is incidentally also why Dogs probably have watched a lot more television in recent years than in the past. They can resolve images up to 75 times a second so to them, CRT television was probably just weird flickery lines.
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