On 2010-12-23 23:38, Paul Ewins wrote:
[...] During the 1/250 sec each spoke travel a little bit further than the distance to the next spoke. This means there are sections where it has doubled up (i.e. both the current and previous spoke have occupied that position during the exposure) so you get the "ghost" spokes as well as the uniform blur.
I think you've got it, Paul. That certainly explains the situations where I've encountered it. And it is a sort of a "temporal moire" pattern thing, which I'm pretty sure is the underlying cause in some form.
Larry, it doesn't seem so much tied to a particular part of the track as it seems to be tied to some larger set of circumstances. I've seen it at different parts of the track on the same and different cars with the same and different sorts of wheels.
If I extrapolate Paul's thinking, it's tied to the combination of the wheel speed, shutter speed, and the specific wheel design (spoke pattern). That seems reasonable based on where and when I've seen it happen.
The panning does other weird things to wheel spokes. I've got dozens of shots where the spokes at the top or bottom of the wheel look straight while the ones at the bottom or top look highly curved. The moving shutter slit could affect these, too.
I'm sure the moving shutter slit does cause some "length telescoping", but I haven't had any racing shots where it was particularly obvious to me.
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