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Old 06-07-2014, 01:52 PM
 
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Some people use their cellphone cameras to take photos/videos of their TV screens or monitors. Sometimes they put them on youtube etc.

I did this once with a tube TV. The still photos had horizontal lines sometimes but you could see most of the image. The videos came out badly, with large black areas. This is because of the refresh rate of the TV.

With a newer TV or newer monitor, it looks better except that for stills there are hazy vertical bands. Why are they vertical? Videos have a slight horizontal rolling lines. If you rotate the camera to different angles, the lines will be diff. I suppose again that's because of the scan rate.

Do all cellphone video cameras have the same scan rate as all flat TVs and monitors?
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Old 06-08-2014, 01:15 AM
 
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Originally Posted by robertpasa View Post
This is because of the refresh rate of the TV.
Camcorders are typically 30FPS and the video on the TV is typically 30FPS so it's not really the refresh rate.

The primary issue with TV is the shutter speed of the camcorder which might be 1/1000 of a second. It's capturing where the point of color is being transformed to another. That 1/1000 of second gets elongated to 1/30 of a second. You'd be able to see this as well except your brain throws out much of that information so it doesn't get overloaded and just fills in the blanks of what it expects you're seeing.


Notice how this appears fairly smooth, your brain actually fills in the blanks of what it expects should be there:






Here it is much slower:




When you have different framerates(or refresh rates) you'll get a flickering but that's the whole frame. For example old home movies are typically 18fps and the home brew method to get these to digital is to point a camcorder at the screen however if you do that the video is going to flicker. If you have camcorder with 24P and film projector with speed adjustment you can just increase the speed until it's 24 FPS, no more flicker. Since film doesn't have this point where one point of color is being transformed to another the rolling is non issue. From there you adjust as needed in video editor to get back to the right speed.

Last edited by thecoalman; 06-08-2014 at 01:24 AM..
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Old 06-08-2014, 02:36 PM
 
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What's the shutter speed of the still cam on a smartphone? Not the video part.
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Old 06-08-2014, 07:34 PM
 
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That's going to vary Robert, try and set it to 1/30 or less if you can. If you can't change the shutter speed you're out of luck.
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Old 06-08-2014, 08:34 PM
 
Location: Somewhere in northern Alabama
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Quote:
Originally Posted by thecoalman View Post
That's going to vary Robert, try and set it to 1/30 or less if you can. If you can't change the shutter speed you're out of luck.
Correctamundo, unless you have a neutral density filter that can reduce light levels enough to force a longer exposure.
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Old 09-12-2014, 03:03 PM
 
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Clarification: when I take a still image of the TV OR film it with video, there are parellel moving bands of lighter or darker.

When I film a computer monitor, there are no bands and looks pretty good. Does the monitor have a different rate from the TV?

(All the above is about the camera in my Android phone.)
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