ELI5- why do screens/lights flicker in videos taken by phone or even yt vids, but not in movies and professional productions?
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Without getting into why the imperfections happen in the first place, professional productions have a post-production step where imperfections are handled. Your phone videos often do not.
Professionals use synchronized cameras, or simply replace the screen displays in post-production with CGI. Professional cameras have lots of adjustments, like longer exposure times, that can give cinematographers more options to capture displays.
Because they change the setting on their camera to compensate.
Screens are just strobe lights that’s are so fast you can’t see them turn off. So the flashing screen on a camera is because you hit the timing just right. A refresh rate for most screens is 60 per second, and cameras are 30 frames per second.
TV and computer screens flicker in videos because they *roll* at the same rate as the screen you are watching on. If its professionally done then it us a simulated screen.
The light flickers at a certain speed, cameras with manual shutters can change so that the flicker doesn’t show. Your phone doesn’t have this.
The rate at which a camera captures images is likely not synced to the refresh rate of a screen or fluorescent light (which also flickers rapidly.)
This is to answer the first part of your question. Screens flicker in video taken by phone or any other video capturing device for one reason only. The frame rate of the video recording does not match the hertz rate of the location that is being shot. I battle this daily with the video team I work with as we shoot in different countries regularly. Most of the younger generation might not know this as most videos these days is consumed online which will play any frame rate and anything progressive or interlaced all the same, but back in the old days, each country had a standard when broadcasting video.
There are several broadcast systems across the world but 2 of the main ones are systems called PAL and NTSC. These are systems based on frame rates that are derived from the cycles of the electricity, aka hertz. For example, in the US, which generally has the supply voltage 120v has a cycle/hertz of 60. This means all lights, screens and anything else that has cycles will flicker at that rate. NTSC is generally considered to be 30fps or 29.97fps if you use drop frame. What is 60 divided by 2? 30. This is the reason NTSC is at the frame rate which means if you are shooting native NTSC, or 29.97 (or 24.97fps for that matter) you could shoot anywhere in the US and there is a good bet if you keep your shutter speed native to the defaults you will not suffer any flickering.
PAL on the other hand (think Europe or some places in Asia) works in locations that have 240v supply voltage. Also 240v in these locations are at 50hz. What is 50 divided by 2? Yes, 25. Guess what the frame rate is of PAL? 25fps. This ensures if you are shooting in a PAL aka 240v supply location, you will also not suffer flickering in most cases. Oddly enough, there is a reason for these systems. Of course this is not a constant, you will have the odd flicker of halogens, also fluorescent tubing and in I most cases, changing the shutter speed slightly will fix this but might not always be the best choice when it comes to low light situations.
General rule of thumb is, shoot the frame rate of whatever the hertz/supply voltage is in the country you are in and you will probably be ok.
Phones tend to shoot in a non fixed variable frame rates. Not all but a majority do so you will find flickering can happen in some recordings but not in others.
But for films, well films have budget, budget pays for very talented people to find ways to sync refresh rates of screens to match the frame rate of the cameras and the everything is perfect.
Old screens like CRT monitors refresh the images line by line from one end of the screen to the other. Because the exposure time of your eyes is longer than the time it takes for the screen to refresh, you see it as a single image.
Video camera shutters are usually set to be much faster, while also capturing 24-60± frames per second. They only take in enough light during the time the shutter is open per frame to get part of the screen exposed. And because it captures many frames a second, it gives that flickering effect due to each subsequent individual frame not matching the previous every time.
If you use a longer shutter speed, it will reduce this effect by letting more light in per frame. You can also synchronize the frame rate of the camera with the monitor’s refresh rate.
Because of shutter speed of some cameras, helicopter blades appear to look like they stay still .
In the old days we had a sync box which would send a timing signal to the camera and any TVs to make sure that the sync rate of the camera matched the TVs. A mismatched sync rate causes flicker.
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