When you look at a tv through a camera?

Question

There appears to be lines and black spots that scroll across the screen vertically. What does the camera see that we can't? Does anyone know?

Answer 1

This is kind of the same effect you see in films, when they show wheels slowing down, which at some point seem to move backwards. If you've never seen this, watch some TV with car scenes, and you may.

Basically, what you see is a difference in discrete sampling. Your eye is working effectively as a continuous input to your brain.. it's not fast, but it's constant. This allows movies and TV to fool your eye into thinking that a series of still pictures shown at 24 to 60 per second is the same as full motion. It isn't really, but as long as we can't tell the difference, it looks like movement.

Your camera, on the other hand, is taking pretty fast samples all the time, whether you're looking through the viewfinder or taking a shot. That sample rate is not necessarily the same as your video source, and even when it is (eg, using a video camera to shoot a TV), they're not synchronized. So the fast capture will sometimes capture a whole picture on the screen, but it's also likely to capture a change between two different pictures, something too fast for your eye to take note of.

This effect is also different with different kinds of TVs. A normal CRT is very easily caught, since the TV tube "paints" the picture on-screen, essentially a pixel at a time, and counts on the phosphors to hold that image. You can't see it fade before it's refreshed, but your camera can.

Modern LCDs change all at once, so they won't show this banding, but may show some visible effect -- a dramatic change will kind of melt from one to the other, if you could film an LCD at hight speed, since the speed of change for any pixel depends on the extent of the change. Very old LCDs are actually scanned rather than changing all at once, and can be caught on camera.

DLP projection systems, on the other hand, change all at once,

but alternate colors at hundreds of times a second. You can't see that, but if you shoot the screen at a high speed, you'll see only one or two colors displayed, not the whole 3, 5, or 6 colors the blend together in your preception.  

Answer 2

Your question makes me to suspect that you better know the answer---am I right? :-)

Well, TV projection is run by 25 frames per sec, whereas camera moves 24 frames per sec. One frame makes the lines when one try to capture TV shows. Since both are in move, so the lines are supposed as if scrolling! Yet, there is a syncronising technology that enables camera to picturise the TV shows as it is.