James,
You are quite right. However, some added thoughts. I guess I was constrained (or
experiencing tunnel vision) by the method as used in my classroom for many years
where we use 33 1/3 rpm turntables and 12 inch diameter (I believe that is what
they are) records ... music records. In the overview of the methods I do mention
that one can raise the shuter speed tested to higher values by using faster
turntable speeds. We also, of course, assume the turntable is turning at stated
RPM. It is mentioned however that questioning the "standard" is yet another
matter so we assume that they are.
As for photographing the moving dot close-up that is a good thought but then
synchronization of the exposure with the time of passage of the moving dot
across the sensor becomes a significant problem. Of course, in a
photoinstrumentation course the students might be asked to come up with ways of
synchronizing the exposure with the passage of the dot! ;)
Just some thoughts ... thanks,
Andy
It's not clear to me what the cause of the inaccuracy is with the use of
a turntable for shutter calibration / measurement above the shutter
speed of 1/3oth second.
If one posits a turntable plate 24 inches in diameter rotating at 78
RPM, then the edge is passing a spot at about 98 inches per second.
If the table plate were constructed with holes in it near the edge with
each hole being about 1 mm in diameter and a very bright light behind
it, then the light streak for the hole would have the following arc
lengths:
1/30th - 3.3 in
1/100th - 0.98
1/200th - 0.49
1/500th - 0.20
1/1000th - 0.10
(values rounded)
Using a macro lens that allows 1-1 or better, you would be looking at a
real image size on the sensor of 1 1/2 times that value ( for DSLRs with
an image magnification factor of 1.5 ).
Surely that would be enough to get quite accurate measurements to at
least the 1/500th second shutter speed.
Just a thought,
James
