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
At 09:13 PM 11/17/2006 -0800, you wrote:
I was asked the following question and I replied below. I thought some of you might be interested in the reply. I recall that Bob Talbot was testing shutters in related ways.
> So, I am wondering how you would go about testing the shutter on a digital camera. I am assuming they are plagued with the same shutter issues that our film cameras are, but we only have pixel values to deal with.
<snip>
1. The simplest test would be to test as done in my high speed class. Photograph a disc rotating at a known speed and measure that resulting arc and divide that by the rotation rate in degrees per second. We use a regular (LP vinyl) turntable and measure up to about 1/100 second. But after about 1/30 the test becomes quite imprecise.
<snip>
I hope this is of some use ... if you discover another approach or if I made a mistake in my suggestions please let me know. BTW, electronically controlled shutters tend to be much more accurate than the older all mechanical ones. I would trust their timing and, in fact, would use them as precision timepieces for certain applications.
cheerio,
andy
