Why do TV cameras pick it up but our eyes don’t?
TV cameras have a much higher frequency response than does our vision system, which includes the eyes, the nervous system and the brain.
The fact that the rapid flicker doesn’t register is a result of the persistence of vision. Here is one website and there are others. http://freeweb.pdq.net/headstrong/Persist.htm
Cameras don’t have this persistence.
Thanks David! Interesting read.
This is true, but only part of the answer.
If persistence of vision was the only difference between looking at a TV screen directly and looking at one through another camera, we wouldn’t see any flicker. After all, your eyes still have persistence, and the second camera and monitor are flickering just as fast as the first monitor.
The thing that causes the flicker when one camera shows a picture of a monitor is that the both the camera and the monitor are “flickering” on and off all the time (just as David Simmons says) but at different rates, or the same rate but out of sync with each other.
That is, a TV or monitor shows a new still picture on the screen about 30 to 90 times a second, depending on your hardware. A standard TV or movie camera captures a new still image about 24 to 60 times a second, again depending on the hardware.
Even if the two devices use the same rate (say 30 frames per second) the monitor may be only part way through displaying a new image at the exact moment when the camera is taking a picture of the monitor. This causes the “black bar” effect commonly seen. And if the camera’s shutter rate is not the same as the monitors display rate, the blank bar will appear to move around, anything from a slow roll up or down, up to jumping around seemingly randomly.
There are ways to sync a TV or movie camera with a monitor or TV, and when this is done, you don’t see any flicker, even though the camera can have a higher “scan rate” than your eyes.
Ugly
This is the “sampling error” phenomenon, or stroboscopic effect. It is the same thing that often causes wagon wheels to appear to go backwards in oaters.
To see this at home with your TV, try placing an electric fan in front of the screen. By viewing the redrawing screen through the spaces between the moving fan blades, you should be able to see a clear flickering pattern.