Meryem Ersoz University of Oregon wrote: (snip) >So I'm kicking the question back to the film list. Can anyone answer this >for me? Why do wheels, when filmed, sometimes look as if they are >spinning in a direction which appears to be the opposite of the direction >which logic tells us they actually must be spinning? Does persistence of >vision have anything to do with it? The wheels appearing to go in reverse is caused by the stroboscopic effect of the camera shutter. The camera shutter cuts off the light while the film is advanced by one frame. If the camera speed and wheel speed are in the correct mathematical relationship, even though the wheel HAS rotated more than one revolution, it may appear to be at a position slightly less than one complete revolution. In this case, the wheel appears to be rotating backwards ( or in the opposite direction ). The persistence of vision is what makes cinema and television work. A series of slightly different images are integrated by the brain into what appears to be a moving image. The phi phenomenon is different. It is what makes a series of still images appear to move. A typical example is the light-bulb festooned billboards outside theatres, cinemas, sideshows etc., where a corrrectly displaying set of bulbs appears to make a moving arrow or something. Tony Saffrey London UK ---- To signoff SCREEN-L, e-mail [log in to unmask] and put SIGNOFF SCREEN-L in the message. Problems? Contact [log in to unmask]