Why is it difficult to photograph a TV picture?

            Engineers have exploited the human eye’s ‘persistence of vision’ to display pictures on television screens.

            The sets flash every frame at a rate of 50 times a second. (This is the frame or refresher rate).That is, each frame appears on the screen for about 20 milliseconds making us feel that the picture is continuous. To see the picture being refreshed on the screen, just wink your eye rapidly and look at a TV screen. A black bar dividing the picture into two should be visible. (This is the simple stroboscopic effect.)

            Cameras have typically a shutter speed of 1/100th of a second, that is, the shutter stays open for only 10 milli seconds. As this time interval is much smaller than the TV frame rate, the shutter does not give enough time for the picture to be displayed next time.

            Hence, the camera freezes the TV picture including the dividing bar and captures it in the film. If you carefully observe the scenes in movies, you will notice that any TV set in them displays the picture with a bar black slowly moving across the picture. To photograph from a TV, use an SLR camera and set the shutter speed to 1/8th of a second (125 ms) for flicker free pictures. (Theoretically, the shutter speed should be just below the frame rate). Use a tripod to avoid hand shake.