Thank you all. Conclusion I gather is that this is a scanning artifact. A derivative of the focal plane shutter effect but through a much narrower "slit" ... in the case of a digital camera essentially a pixel wide so the moving image edge is very sharp. As the image plane is scanned the prop blades pass by the moving scanning line several times and this gives rise to the fragmented nature of the prop. I think I may try to do this with a film camera under controlled conditions so I can see "in slow motion" the generation of such an image. Or maybe one of you can do a slow-motion computer simulation! ;) andy
