I think Paul gave the right explanation, or at least that the explanation I head the moment I read the answer. My estimates gave similar numbers, but I didn't have time to write a message, so I was sure somebody else would write it. (That's what professors often do when teach a class: they let other students to answer the question raised during the class. :-) Sorry, Paul. :-P )
Just in case, the number of spokes is indeed 15: http://en.wikipedia.org/wiki/File:Neckarsulm-AudiForum-Audi-R10-TDI.jpg Within 1/250s the tire should've made just over 1/15th of a turn. The reason I decided to follow up on Pauls response is that I wanted to point out a few variations of this effect. An interesting shot to compare is this one: http://www.flickr.com/photos/j-c/2669414376/ The blur is broader with a slightly shorter exposure (1/200s, according to the exif) and probably higher car speed. I should note that a similar effect can be expected when the tire makes not about 1/15th of a turn, but also 2/15th, 3/15th, etc. (N/15), but the contrast will be lower, and it probably wouldn't work beyond N=3 (or maybe even 2, depending on the light conditions). In this shot made at 1/60s, the spokes are mostly blurred, but there is a "negative" spoke (anti-spoke): http://www.flickr.com/photos/fabiobombelli/2455925901/ I suspect in this case the tire made somewhat less than N/15th (2/15th or 3/15th) of a turn, - and so, the darker portion corresponds to the part where the spokes appeared N-1 times while the shutter was open. Then now, when I had time to look at it in detail I decided to estimate the car speed more precisely I found the tire size, but I was not able to decipher it: http://en.wikipedia.org/wiki/Audi_R10_TDI * Front: 33/68-18. * Rear: 37/71-18. I understand that the rims are 18". What's the first (and the second) number(s)? In the two systems of notation that I know it can be either tire width or its height. Neither of them makes sense in inches (too large) or in mm (too small) In those two systems, the second number would be the ratio of the sidewall height to the width or the width, respectively. The only sense I can guess is if the first two numbers were in cm. I am sure any of the car enthusiasts on the list would be able to explain this easily. - I'd appreciate that. Igor On Dec 23, 2010, at 8:38 PM, Paul Ewins wrote: > 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. -- 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.
