OK, I know there are some folks around the list that know a lot more
about the physics, optics, and other science surrounding our hobby than
I do. And I'm asking their help in understanding something.
In the following photo, take a specific look at the way that the wheels
and in particular the spokes are rendered. It's a digital capture, but
I've seen the same effect in film captures.
http://www.facebook.com/photo.php?fbid=119672688098162&set=a.114665221932242.11964.100001662949948
Specifically, notice that the spokes, especially on the rear wheel,
appear mostly as blurs of grey. But also notice that there are specific
highlights that look like slightly blurry spokes themselves. The number
of these artifacts in the photo is identical to the actual number of
spokes on each wheel. I specifically checked after having noticed this
effect in previous shots.
But I don't understand how they get there. I'm suspecting is some sort
of "temporal moire" thing going on, but I can't envision the mechanism.
And that's the help I'm asking. I'm hoping one of you can explain the
physical mechanism that's leading to those blurry spokes in the wheel
instead of just showing a less "peaked", more uniform grey blur.
The shutter speed was 1/250, which is intentionally slow enough to give
the wheels and background a blur while giving me a chance to get a sharp
shot of the body while panning (I shoot almost all of the moving car
shots on the track at 1/250, if you can see the wheels in the shot;
1/125 if it's a slow corner). The car was going about 100 miles an hour
and accelerating as the shot was taken.
--
Thanks,
DougF (KG4LMZ)
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