Andy,
Fine, now explain why the pin heads don't fall on a straight line extrapolated to (a)(the) center of rotation. The nearest pinhead to the center of rotation (which isn't indicated) appears to be moving at a higher angular velocity than the others, but even the more outboard pinheads don't appear to be moving at the same angular velocity, an impossibility. The starting point is kind of hard to discern. it appears that they weren't all lined up in a straight line. Curtain movement should place them in a curve, not a sine wave, shouldn't it? Also, only the final position is fairly well defined, but not the starting point. Even then, the diameter of the pinhead on one track I measured is about 3 mm (in my roughly 12cm by 25cm printout), while the track is about 67 mm. This gives a precision of only about 4 percent.
At an exposure of 1/6s, 33 1/3 rpm gives an angular rotation of 200 degrees/s. Thus an exposure time of 1/6 s should give an angular rotation of the pinheads of 33 1/3 degrees. My measurements of the angle give about 30 degrees, but they are imprecise because I can't tell the starting point precisely enough.
Would a half black - half white disc do a better job? Do you let your students come up with their own method to measure the shutter speed this way? Or do you tell them how to do it? This one would be useless at a shutter speed of 1/4000s, I think. How do you do that?
Finally, what grade would you give this effort? On a scale of A to F, say? I would have required my students (engineers, not photographers!) to present an error analysis with uncertainty bounds and a proposal for doing it better the next time and maybe awarded it a C-, especially if I knew that the shutter speed was 1/6s within a percent of so.
I've assumed that there was no wobble in the turntable.
Please know that much of this is tongue in cheek! I liked the other photographs, but I'm not an artist so I won't comment. They are all much better than I would have done!
Roger
Very interesting and I apologize for not being able to find the time to discuss fully all the photographs in the gallery and all the technical ramifications of the image/process illustrated by Bob but just wanted to say that in my high speed photography course one of the first projects the students complete is one of calibrationg a shutter against a standard (331/3 rpm turntable and a b&w TV set)
From something as simple as this a whole lot of "technical" spinoffs canbe pursued.
I particularly like the fact that even though the total exposure is the same everywhere the exposure for moving objects is not the same as for stationary ones. In fact, the moving subjects become their own shutters in a sense. Another approach is to note that the pinhead specular highlights are not the same in density ... the energy that would arrive at a single spot on the film if the pinhead were still is further and further spread out over a larger arc as one moves away from the center of the ttbl. That density loss could be related to pinhead velocity - I call that sensitometric velocimetry (not a very practical thing to do but makes brain work some). For one one needs to account for the increase in noise picked up from the less than perfectly black background ... hmmm, my head hurts!
Anyway, IMO very nicely done - better than my students do it!
click! Andy
PS: in brief I find the composition/content/treatment of the photographs by Siegel, Snarski, Cigirgan and Davis particularly well done.
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