I'm not sure that I'm answering your question correctly, but I threw a whole lot of assumptions at a spreadsheet and came up with the conclusion that in that 1/250 sec the wheel rotated around 1/10 of a revolution. If I am counting right there are 15 spokes in the wheels. 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.
Paul On 24/12/2010, at 2:39 PM, Doug Franklin wrote: > 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) > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
