why when we take a pic from a crt monitor it has noises?(doesnt have a good quality)?

Question

i want to know scientifical reasons for this and what structure of crt monitors make this happen?

Answer 1

CRT monitors work by scanning a small beam of electrons across the screen that is covered with phosphor dots. When the electron beam hits the phosphors they absorb the energy and then give it off as light. The phosphor will continue to give off light after the electron beam has moved on, but the brightness of the light drops rapidly and exponentially.

So what you have is a single bright dot of light, with a long tail scanning across the front of the CRT in a series of lines from left to right, and the lines running top to bottom. When the electron beam gets down to bottom right, the monitor turns it off and quickly gets it back up to top left to start all over again. The time it takes to do this is called the vertical blanking time.

However, due to the way your eye and brain work, if you move the beam fast enough your brain sees a static image.
60 Hz refresh is marginal for this - if you look at something next to the monitor you will see the image flicker. At 75Hz only a few people see the flicker at 85Hz effectively no one can see it.

However, the camera sees what is there. If your shutter speed is faster than the refresh rate then the electron beam can not complete the full screen in the time that the shutter is open and the camera only sees part of the screen lit up. If the shutter speed is slower than the refresh , but faster than 2xrefresh then it sees the entire screen covered once, but part of the screen covered twice and that part of the screen will look brighter in the image.


You can either: Set the shutter speed at or close to the refresh time, and take lots of pictures so that any missing image or extra image falls into the vertical blanking time.

Or: Use a tripod and set a very long shutter speed so that multiple scans are captured and and extra brightness of a partial scan is lost in the image from the multiple full scans.

Answer 2

Great answer above. If your monitor has a refresh rate of 60Hz, you'll want a shutter speed of 1/60 (one sixtieth of a second), 1/85 for 85Hz and on. I would suggest using 1/20 for 60Hz, and 1/30 for 85Hz however (so it your shutter stays open long enough to take ~three full scans.) Tuning down your aperture setting might help too, so it captures less light at once (if you have an A/S setting on your camera, set it to S for shutter priority, it'll set the aperture size by itself)