Speed Graphic Focal Plane Shutter

[Mattk]

Doh! Just realized the power came from the ignition system. I was thinking strobe light. Whoops.

[Jim Graves]

It finally occurred to me to check the shutter tester against a proven focal-plane shutter. ....... I think the wave shape seen from a photodiode through a sound card is just too hard to interpret with sufficient accuracy at speeds of 250 and higher, and perhaps even 125 and higher. ....... [parts of quote deleted].

Rick ... I came to the same conclusion about the inexpensive speed testers.

I have tried two of the photo transistor shutter testers regularly available on ebay ... one is identical to the build-it-yourself version from plans on the internet [uses a free download program for timing on your computer][$40 range] ... the other a more recently available Chinese manufactured tester with the readout right on the unit [$120 range.]

I tested both on 3 relatively modern leaf shutter LF lenses that had just been professionally timed by California Precision Services here in Sacramento. I warmed up the shutters with at least 20 releases prior to testing and took 10 readings at each speed. The results were very accurate up to 1/100 at which point they began to vary by large percentages [growing larger as the dial speed increased] ... in the 1/250 and 1/400 range they both indicated at least 25% slower than the professional results on all 3 lenses. A small sampling admittedly but very consistent so I would hazard a guess that either the photo transistors or the circuitry limits accuracy at higher sutter speeds.

I'm curious what other members here have experienced with these testers.

[rdenney]

Rick ... I came to the same conclusion about the inexpensive speed testers.

I have tried two of the photo transistor shutter testers regularly available on ebay ... one is identical to the build-it-yourself version from plans on the internet [uses a free download program for timing on your computer][$40 range] ... the other a more recently available Chinese manufactured tester with the readout right on the unit [$120 range.]

Struan and I were having this conversation in another thread just yesterday, and what follows is out-loud thinking. The rise time of typical photo-diodes, when properly driven, is in the tens of microseconds--easily fast enough for this application. The problem is the sound card being used as a data sampler. The sound card is designed for audio, and given that all audio lives in the frequency domain, the sound card will filter out direct current. The current passed by the diode (as pulled by the resistor) will be low when there is no light, and high when there is light. Smaller resistors pull more current and more current means more power and a faster rise time, but it also means pushing the diode closer to its power-handling limits. Most of these home-built testers use high resistances to keep the current flow fairly low.

But when the current rises, so does the output voltage of the diode. And that's what the sound-card reads. But even high-end sound cards roll off response below about 30 Hz using something like a capacitor on the input, which filters out DC and also puts a "swoop" in the rising and falling waveforms as the diode is switched. One way to cure that would be to feed high-frequency audio to the diode instead of DC, using an oscillator. The frequency of oscillation would have to be fast enough to be negligible to the shutter speed being tested, but slow enough to be within the frequency response of the sound card and also slow enough to not be canceled out by the rise and fall time of the diode, if that is possible.

Or, a properly driven diode could be read by a device that doesn't filter DC, such as a data acquisition system for the PC, or a storage oscilloscope. These are a lot cheaper than they used to be, but they are still 20+ times that cheaper tester on ebay unless you know how to scrounge, or write your own acquisition software.

I suspect the Calumet tester, which I know will measure high speeds accurately, couples a fast photo-diode to a circuit that triggers a counter on the first positive change of voltage, and again on the first subsequent negative change in voltage. Not hard to do with a diode detector. It then couples that output to a high-speed timer. The Calumet tester will trigger on a single pulse of a fluorescent light, and even the AC pulsing of an incandescent light will mess it up. The detector is sensitive and the light has to be at just the right intensity. If the more expensive tester is like the Calumet tester, then try it again with a battery-powered flashlight (with a real bulb, not LEDs) and arrange the test so that the light is just barely bright enough to trigger it. You might get more accurate results.

Rick "who might give this project to a local ham radio group to see if one of them will whitewash the fence" Denney

[jp]

Sounds like you need to hook it up to an O-scope instead of a sound card. It would be simple to measure the width of the electrical pulse with that.

[rdenney]

Sounds like you need to hook it up to an O-scope instead of a sound card. It would be simple to measure the width of the electrical pulse with that.

Yup. But my scope is not a storage scope, and getting one of those would change the cost equation.

Rick "already subject to wifely scrutiny regarding test equipment" Denney

[Jack Dahlgren]

Or, a properly driven diode could be read by a device that doesn't filter DC, such as a data acquisition system for the PC, or a storage oscilloscope. These are a lot cheaper than they used to be, but they are still 20+ times that cheaper tester on ebay unless you know how to scrounge, or write your own acquisition software.

I suspect the Calumet tester, which I know will measure high speeds accurately, couples a fast photo-diode to a circuit that triggers a counter on the first positive change of voltage, and again on the first subsequent negative change in voltage. Not hard to do with a diode detector. It then couples that output to a high-speed timer. The Calumet tester will trigger on a single pulse of a fluorescent light, and even the AC pulsing of an incandescent light will mess it up. The detector is sensitive and the light has to be at just the right intensity. If the more expensive tester is like the Calumet tester, then try it again with a battery-powered flashlight (with a real bulb, not LEDs) and arrange the test so that the light is just barely bright enough to trigger it. You might get more accurate results.

Rick "who might give this project to a local ham radio group to see if one of them will whitewash the fence" Denney

I just ordered one of these:

http://www.sparkfun.com/datasheets/S...230R-LF-e3.pdf

and am planning to hook it up to an arduino or other microcontroller to log data and calculate exposure. I think it should be possible to catch the front edge of the shutter opening and integrate over the entire open time. Then compare against the peak value in order to get an effective shutter speed regardless of f/stop.

I'll update on progress as soon as it gets here and I can build a circuit.

Once I get that working it can probably be made into a sensitive light meter. With a time series of readings it probably could calculate flash exposure and other stuff as well. Once you have the data there are tons of things you can do with it.

-Jack

[rdenney]

I just ordered one of these:

http://www.sparkfun.com/datasheets/S...230R-LF-e3.pdf

and am planning to hook it up to an arduino or other microcontroller to log data and calculate exposure. I think it should be possible to catch the front edge of the shutter opening and integrate over the entire open time. Then compare against the peak value in order to get an effective shutter speed regardless of f/stop.

I'll update on progress as soon as it gets here and I can build a circuit.

Once I get that working it can probably be made into a sensitive light meter. With a time series of readings it probably could calculate flash exposure and other stuff as well. Once you have the data there are tons of things you can do with it.

-Jack

And I just ordered one of these:

http://pdamusician.com/dpscope/index.html

It's more than I paid for my Tektronics portable, but it has storage and plenty of bandwidth for this and for spectral analysis of music. It's no good for the RF stuff (except at audio frequencies), but then I don't need a storage scope for that and the Tektronics scope works fine.

Rick "don't tell my wife" Denney

[BetterSense]

Once I get that working it can probably be made into a sensitive light meter.

I have made several light meters with these chips. They are very easy to implement with the digital output but downright require spectral response filtration.

You can see them on my very embryonic website

chazmiller.com/projects/lightmeters.html

[Jack Dahlgren]

I have made several light meters with these chips. They are very easy to implement with the digital output but downright require spectral response filtration.

You can see them on my very embryonic website

chazmiller.com/projects/lightmeters.html

Interested in sharing the code? I'm heading down the same path and could use a lift.

[kidslong]

I have experience with a Calumet tester that I borrowed from my friend a few years ago