In response to this question posted by Meryem Constance Ersoz: >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?< I suppose that the effect of wheels spinning backwards is created more by the mechanics of motion picture photography than by persistence of vision. Maybe the best way to answer the question is to use a simplified example. Imagine you had a disc with a single line drawn on it from the centre to a point on the circumference. You then set the disc spinning at a constant rate of one revolution per minute. Now suppose you were filming this disc spinning using a camera that made one exposure each second. As the line would be in the same position each time an exposure was made, it would appear to be stationary rather than spinning on the resulting film. To achieve the 'backward' effect, you would simply need to speed the camera up slightly to, for example, 0.95 seconds between exposures or, alternatively, slow the disc down. Now the disc would be completing slightly less than one revolution between exposures and (hope you can imagine this!) would therefore appear on film to be moving in the opposite direction to which it is actually spinning. To apply this in practice, you simply have to speed the whole thing up - 24 frames a second and a wheel that rotates perhaps only 23 times a second. Hope that gives a solution to at least part of the problem. It would be easier to draw diagrams but, unfortunately, that's not too easy to do on email! Iain Duncan [log in to unmask] ---- To signoff SCREEN-L, e-mail [log in to unmask] and put SIGNOFF SCREEN-L in the message. Problems? Contact [log in to unmask]