why is it that when you take a picture of a moving fan the blades appear to be going in the opposite direction? Let me add to fanoaa answer, it can happen also with naked eye, this is connected to the speed our brain process the images which mean we actually see the movement as set of stills pictures although we think of it as continuous movement. This phenomena is called aliasing (in signal processing) which means certain input can be interpreted as different input. this should only happen on a digital camera or a video camera.
the reason is, that the camera is displaying say, 24 frames per second, and the fan is rotateing at 23 revolutions per second. thus, when each frame comes around, the blades are slightly earlyer in there revolution than they where in the last one. this happens every frame, and the result is, it appears to be rotating backwards slightly. (the reverse can be true, they appear to be rotoating the same direction, but slowly, this is caused when the blades move faster than the frame rate, say 25)
somtimes, the rate the blades spin is exacly the same as the frame rate of the camera, that makes them appear to hold still. (there recently was a video on the internet of a helicopter where the rotors where rotoating at the framerate of hte camera, so it looked like the helicopter is flying with the blades still.)
hope that answers it. |
