Andy:
To overcome the synch problem, I posited many holes on the edge,
maybe one per inch or so. Only one light directly beneath the camera
and with a width of at least 1.5 inches so the hole would be
illuminated for the entire exposure period.
With modern digital cameras, between 3 and 8 frames per second would also help.
Even with a 12 inch platter, you should be able to get a good measure
for up to about 1/200th second.
The values would be ( approximately for 78 RPM, a speed us old
codgers will remember from before the mid-point of the last century <grin> )
1/30th - 0.640
1/100th - 0.192
1/200th - 0.096
1300th - 0.064
1/500th - 0.038
1/1000th - 0.019
Using an APSC sized sensor and a 1-1 close-up, actual image size
would be that shown but in 35mm frame sizes would be about 50% longer.
So 1/100th would give the equivalent of 0.3 inches, enough for a
reasonably accurate measure. With multiple samples, the values for
1/200th could be determined.
Cheers,
James
At 11:19 AM 11/19/2006 -0800, you wrote:
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
James Schenken
