Interesting Idea

[SteveH]

All,
Late last night when I was looking on some info I came across this:
Dead Link Removed

Perhaps more trouble than its worth, but a cool project on a rainy day.

 

[sajianphotos]

I have seen something like this before and I think it's a cool idea. Doesn't look like that much trouble to me, but then I'm an insatiable tinkerer.

 

[RGyori]

I came across that and other similar web pages late last year. I used a variation of the Radio Shack approach and built a unit that was connected to my PowerBook to test 4 lens/shutter assemblies (2 Rodenstock, 1 Schneider and a Nikon). Interestingly enough -to me at least!- the slower speeds up to about 1/125th of a second were well within 10% but I started to see some significant variations at 1/250 and 1/500 (or 1/400) with errors up to 50% on one shutter! And these are relatively new and well maintained shutters.

Took some fiddling but it worked out fine and the results were repeatable and consistent.

Bob

 

[SteveH]

So Bob...Was this an issue with the shutter, or an issue calibrating the calibrator ?

 

[RGyori]

Steve-

The issue would appear to be the shutter... or my methodology. The software/hardware setup simply uses the computers clock rate to "slice" time with the software measuring the intervals. No calibration is necessary to my knowledge. (Caveat: Knowledge in my case is entirely relative!).

The challenge proved to be that at higher speeds, say 1/125 and faster the term "square wave" becomes advisory only. The shoulders of the waves were increasingly rounded and of course closer together. I had to zoom in to get a better look at the wave structure thus making an accurate reading more difficult. Of course for the slower speeds the shoulder of the wave forms were more nearly square and the spacing nice and wide.

I suspect the photocell response time might not be quick as needed for the shorter intervals. At least that's my story for now.

Aha! I just had idea. I used an adjustable incandescent desk lamp as my light source... and I just now realized that it cycles at AC 60 Hz. Could this lead to inaccuracies at shorter intervals? Perhaps I should repeat with a constant light source such as a flashlight. Hmmm.

Bob

 

[SteveH]

Bob,
Well, I was thinking of the same issue with a 60hz source...But I was going to keep it to myself until I found otherwise. I was going to use a battery powered LED light source and see how that would work out.
I really don't think the photocell would have much of a problem.

 

[avandesande]

incandescent lamps don't cool quick enough for this to be an issue. it is with flourescents though.

 

[RGyori]

avandesande-

I might be out of my depth a bit here (and not for the first time!) but...

I have an old B & O phonograph turntable the instructions for which specifiy that the use of an incandecent light source directed at the platter in order to adjust the rotational speed. There's a circular arrangement of tightly spaced radial lines near the center of the platter that apparently rely on the stroboscopic effect of a 60Hz light source so that when the array of lines stop moving you have adjusted the platter speed correctly. I therefore presume that the light must be turning on and off at 60Hz.

In any case I will get back to my shutter testing sometime in the near future, this time with a battery powered light source. I'll see if the results are measurably different. Onward.

Bob