Measuring shutter speeds with the Photoplug

ic-racer · Jan 24, 2024

tjwspm said:
Very interesting! That's what was missing from this whole debate. A seemingly simple and precise device to check the measuring equipment.
How is the disc driven? It has to be speed-controlled, right? How did you calibrate it?

A common drill spins the wheel. Math was used to calculate the circumfrence at which the velocity was the same velocity as the focal plane shutter. The slits were made 'pie-shaped' with projection slide cropping tape, like a cine camera focal plane shutter, so that if the measuring point was not exactly at the 'correct' circumference, the shutter speed will not change.

The RPM of the drill was measured by attaching a flag to the drill and measuring the time for ten spikes on the screen and dividing by ten.

During the test recordings of the waveforms, the drill speed was double checked, again measuring the time for ten revolutions and dividing by ten.

Nominal speeds are printed on the wheel, but to be exact this wheel shown gives 1/240, 1/480, 1/960 because the cameras I was testing synch at 1/60 (not 1/64) and then half the frame distance with each increment in shutter speed.

But for testing a shutter tester you can make the slits any width and any RPM as long as you know the slit width and RPM you can calculate the rest.

tjwspm · Jan 24, 2024

Thank you for the instructions. Great!
I will definitely recreate this. It is good practice to first verify the measurement setup before starting measurements.

koraks · Jan 24, 2024

Andreas Thaler said:
I haven't looked into this in detail, but isn't ohmic (transition) resistance a variable in alternating current resistance (impedance)?

The signal runs via plug and socket?

Yes, but it's not a significant factor in this particular case. If contact resistance would be an issue, the reported problem would be wildly erratic output or no output at all. So yes, contact resistance is indeed part of overall impedance because the contact is in series with the input, but there's no indication that this is the problem here.

Chan Tran said:
A low input impedance which the mic input usually is, is around 150 to 200Ω. Dirty contact can add a couple of ohms but that is not significant. A line in is typically 1 to 5kΩ.

Thanks; I stand corrected; I thought that headset mic's were high impedance devices. They're indeed low impedance. Line level is indeed 1k ~10k typically.

titrisol said:
Can you explain the ringing part?

It's barely relevant, really; the conclusion remains "try a different input/device and see what happens". But if you're wondering, there's a good sampler here: https://en.wikipedia.org/wiki/Ringing_(signal) I'd take it from there if you're interested, but as said, from a practical perspective, the advice stays the same.
Have you tried plugging the photoplug into one of the inputs on your computer?

Chan Tran · Jan 24, 2024

koraks said:
Yes, but it's not a significant factor in this particular case. If contact resistance would be an issue, the reported problem would be wildly erratic output or no output at all. So yes, contact resistance is indeed part of overall impedance because the contact is in series with the input, but there's no indication that this is the problem here.

Thanks; I stand corrected; I thought that headset mic's were high impedance devices. They're indeed low impedance. Line level is indeed 1k ~10k typically.

It's barely relevant, really; the conclusion remains "try a different input/device and see what happens". But if you're wondering, there's a good sampler here: https://en.wikipedia.org/wiki/Ringing_(signal) I'd take it from there if you're interested, but as said, from a practical perspective, the advice stays the same.
Have you tried plugging the photoplug into one of the inputs on your computer?

I think the ringing because the sound card input which is intended for audio would not accept DC input. It has a capacitor to block the DC and the charging and discharging of this capacitor causes the ringing.

koraks · Jan 24, 2024

Chan Tran said:
I think the ringing because the sound card input which is intended for audio would not accept DC input. It has a capacitor to block the DC and the charging and discharging of this capacitor causes the ringing.

The cap surely plays a role in it, but so does the input impedance. With a lower input impedance, the ringing would be damped much more, I'd expect.

titrisol · Jan 24, 2024

koraks said:
--snip snip---

inside PP there are 2 4600ohm resistors (yellow-blue-red-gold) and a BPY62-5 PT

koraks said:
Have you tried plugging the photoplug into one of the inputs on your computer?

I haven't yet, only with the iOs app.
I will try tonight and check with audacity

titrisol · Jan 24, 2024

reddesert said:
I have a couple of homemade knockoffs of the Photoplug. The innards of mine are simply a phototransistor and a resistor. I have: an infrared phototransistor (a PT in a black casing), which doesn't work well with optical light unless the light is really bright (I use an LED bike headlight, or the sun); and an optical PT in a clear casing.

Although the switching time of virtually any PT (probably tens of microseconds) should be much faster than needed to measure shutter speeds of several millisec, I get different results with my two PTs. The infrared PT toggles "on" for the entire shutter open time and then "off". The optical PT has a pulse and decay when the shutter opens, and then another pulse and decay when the shutter closes, a similar behavior to Andreas's plots in the linked X-700 thread.

I think that this is likely due to some interaction of the phototransistor properties and the capacitance (or if you like input impedance) of the phone's microphone input. I also think that it may behave differently if you plug it into a 1/8" to Lightning or USB adapter, vs directly into a 1/8 jack.

Keep in mind that audio circuits expect to reproduce an AC signal, not a DC offset, so the decay behavior may be affected by the audio circuit design.

Yes, that is my experience

Chan Tran · Jan 24, 2024

I use a variable light source. At the moment the light source is LV12. The aperture of the camera is set at f/8 and the shutter speed indication in the viewfinder is 1/30 (ASA100) thus the indicated shutter speed is low. Now with the sensor reading the shutter curtain and feed the signal into an USB based oscilloscope I have the pulse width is 18.92 milliseconds or 1/53 of a second. Which is close to the 1/60 speed that it's supposed to be. So the problem is the actually shutter speed is close to what is should be but the indication is low. So I have to adjust the galvanometer movement and not the signal feeding the shutter control circuit.
The sensor uses 2 fiber optic cable, one to shine a visible red light onto the target the other is feeding the light reflected back from the target and feed the amplifier.

Chan Tran · Jan 25, 2024

koraks said:
The cap surely plays a role in it, but so does the input impedance. With a lower input impedance, the ringing would be damped much more, I'd expect.

I am quite sure it's the DC blocking capacitor. I feed a 1Hz square wave into the input of the sound card and I have the same wave form as Andreas did.

tjwspm · May 22, 2024

ic-racer said:
A common drill spins the wheel. Math was used to calculate the circumfrence at which the velocity was the same velocity as the focal plane shutter. The slits were made 'pie-shaped' with projection slide cropping tape, like a cine camera focal plane shutter, so that if the measuring point was not exactly at the 'correct' circumference, the shutter speed will not change.

The RPM of the drill was measured by attaching a flag to the drill and measuring the time for ten spikes on the screen and dividing by ten.

During the test recordings of the waveforms, the drill speed was double checked, again measuring the time for ten revolutions and dividing by ten.

Nominal speeds are printed on the wheel, but to be exact this wheel shown gives 1/240, 1/480, 1/960 because the cameras I was testing synch at 1/60 (not 1/64) and then half the frame distance with each increment in shutter speed.

But for testing a shutter tester you can make the slits any width and any RPM as long as you know the slit width and RPM you can calculate the rest.
View attachment 360940

Finally, today I tested the Photoplug according to your idea.

In combination with the Shutter Speed app it delivers good accuracy. If you want to know how I tested and what specific results I got, you can read it here:

Checking shutter speeds of Minox 8x11 cameras - Moments of Now

So let’s measure the shutter speeds of our Minox. But how? Read my simple method to accurately measure Minox shutter speeds here.

moments-of-now.com

loccdor · May 22, 2024

There are also quite a few phones with native extreme slow motion video recording capability now.

ic-racer · May 22, 2024

Chan Tran said:
The sensor uses 2 fiber optic cable, one to shine a visible red light onto the target the other is feeding the light reflected back from the target and feed the amplifier.

Great idea. Did you make the cable? I need the same for my Minox tester.

Chan Tran · May 22, 2024

ic-racer said:
Great idea. Did you make the cable? I need the same for my Minox tester.

No both the sensor and fiber optic cable are off the shelf item. I used them at work and they would cost something like $200-300 for the set but I bought them from Ebay from China for like $40 for the set. The fiber optic is Keyence FU-66 I paid $15 with free shipping. The sensor is Keyence FS-V31P and I paid $25 with free shipping. The only thing is that the response time is 30 microseconds which is just barely OK to check speed 1/2000 or so.

Measuring shutter speeds with the Photoplug

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Checking shutter speeds of Minox 8x11 cameras - Moments of Now

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Measuring shutter speeds with the Photoplug

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