Why do cameras mess up taking pictures of screens/tv's?

The picture on the tv is changing at a rate of 60 times a second (sometimes more, sometimes less). The camera is liable to catch 2 (or maybe more) different frames in the same exposure. The black line will be where the image refreshes and for a moment doesn’t show anything. Normally this happerns to fast for the eye to detect.

@Lightlyseared even when its taking a picture of a computer screen with a still image its sycling like that?

Yes.

@XOIIO XD

@Lightlyseared is correct. The camera gets a very accurate snap of what’s on the screen precisely when the shot is taken – which is often part of one frame and part of the next. Our eyes smoothe it out to look even.

I was going to say, it’s the differing frame rates of a computer screen vs. a high-end video camera or film.

An LCD computer screen refreshes at 60 frames per second.
35 mm film rolls at 24 frames per second.
An HD video camera’s probably about 24–30 frames per second.

huh. I figured there would be some sort of feature that slowed it doen when it wasnt playing video.

The refresh rate of a monitor is so regular that it’s theoretically a trivial matter (compared to, say, breaking an asynchronous cypher) for someone parked in a van outside your house to pick up the electromagnetic signal that your computer sends to the monitor. This is one of the reasons why when you type a password it’s not displayed on the screen.

(Admitedly it was probably a lot easier back when every one was using CRT screens as I guess the electron gun produces a lot more noise)

it is the refresh rate of each device (as explained above) to get an example of this you can see with the naked eye, look at the tires on a rolling car etc and notice when they appear to be still or even turning backwards. That is because your eye captures images at around 16 frames per second, and when the wheel is turning at a rate where the spokes are in the same place 16 times a second, it looks like it is standing still. Your TV refreshes at 60 frames per second so your eyes blend the images together so you see a smooth image.

@DrBill Yeah, thats always neat XD

The way you get around it is by taking your photograph at 1/ 30th of a second or slower. The lines don’t appear with slower shutter speeds.

@Photosopher don’t you get double-drawing unless it’s quite a bit slower than ⅓0 second ?
I.e. at ½0th I think you get a full frame drawn plus a fraction of the next ?
At 1/10 of a second you’ll get three frames and might be getting a more smoothe result ?
And better at every even multiple of ⅓0 second I think.
At those speeds better bring the tripod !