Measuring shutter speeds with the Photoplug

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Andreas Thaler

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I'm currently renovating a Minolta X-700 and during the work I measured the shutter speeds with the Photoplug.

The Photoplug consists of a light-sensitive measuring cell that is controlled via an app on the smartphone.

Here you can go directly to the practical report, which describes a series of measurements with the tool:

Post in thread 'Minolta X-700: A renovation project'
https://www.photrio.com/forum/threads/minolta-x-700-a-renovation-project.203947/post-2763053


The main story:

 

titrisol

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you were able to measure beyond 1/200? I'm impressed!
 

koraks

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Your phone must have a very quick response, I use it in an old iPhone4 and anything faster than 200 gets lost in noise

This depends on the light source as well. If you look at the plots in Andreas' example, you can see some wiggly noise which is likely the switching frequency of the LED driver. There are plenty of light sources (including LED) that will flicker at twice grid frequency (so 100Hz / 120Hz) and those may give problems especially with faster shutter speeds.
 
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Andreas Thaler

Andreas Thaler

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Interesting!

I think my light box still has fluorescent tubes installed, it's older. But I'm obviously lucky that it works.
 
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Andreas Thaler

Andreas Thaler

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What would be the ideal light source for such measurements?

By the way, my iPhone is no longer the newest, but I'll use it until it's no longer supported. The battery capacity is still at 95% despite daily intensive use.
 

Chan Tran

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What would be the ideal light source for such measurements?

By the way, my iPhone is no longer the newest, but I'll use it until it's no longer supported. The battery capacity is still at 95% despite daily intensive use.

A constant light source (no flickering) with color temperature of about 4700K. Variable between LV 9 and LV 15. That way when you set the light brightness to a certain LV then you can test the camera auto exposure time. This can be different from manual set time.
 

titrisol

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This depends on the light source as well. If you look at the plots in Andreas' example, you can see some wiggly noise which is likely the switching frequency of the LED driver. There are plenty of light sources (including LED) that will flicker at twice grid frequency (so 100Hz / 120Hz) and those may give problems especially with faster shutter speeds.
I have used photoplug for leaf shutters and going faster than 200 is a challenge
I was my understanding that focal plane shutter were tricky, so I tried in a recently CLA camera and I can't get peaks as well defined at 250
 

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koraks

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@titrisol that's pretty pronounced 'ringing' as it's often called in electronics. I'm not familiar with the innards of the Photoplug, but the first thing that comes to mind is that it matters a lot what the input impedance of the phone/tablet is. This is of little help to you, I imagine, but from a practical perspective, you could try plugging it into a different type of device and see if you get the same kind of results. My expectation is that a lower input impedance device will give better performance. This means that the device may work better on a line input than a mic input.
 

Chan Tran

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Which type/frequency?

In Austria we have a mains frequency of 50 Hertz (230 Veff AC).

There are almost no light bulbs anymore available, LEDs with the corresponding electronics to switch remain.

I got this from Home Depot but I guess you could find something similar in Austria.
I was surprised that the color temp is constant at 5200K even when I dimmed it and yet it doesn't flicker. I use the Minolta booster sensor connected to an oscilloscope to check.
 
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Andreas Thaler

Andreas Thaler

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I got this from Home Depot but I guess you could find something similar in Austria.
I was surprised that the color temp is constant at 5200K even when I dimmed it and yet it doesn't flicker. I use the Minolta booster sensor connected to an oscilloscope to check.

Thanks!
 
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Andreas Thaler

Andreas Thaler

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@titrisol that's pretty pronounced 'ringing' as it's often called in electronics. I'm not familiar with the innards of the Photoplug, but the first thing that comes to mind is that it matters a lot what the input impedance of the phone/tablet is. This is of little help to you, I imagine, but from a practical perspective, you could try plugging it into a different type of device and see if you get the same kind of results. My expectation is that a lower input impedance device will give better performance. This means that the device may work better on a line input than a mic input.

To lower the impedance, it might also help to clean the connector on the iPhone and the plug.

My iPhone refuses to charge every few months, then I clean both with electronics cleaner and it works again. If the contacts are not clean, this creates additional resistance.
 

koraks

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To lower the impedance, it might also help to clean the connector on the iPhone and the plug.

No, this doesn't change the input impedance of the device. Contact resistance is a different thing from input impedance. There's no sign of a poor contact in the scope plots of @titrisol.

To clarify, it evidently doesn't hurt to clean the connector. But it's not going to solve this particular problem.
 
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Andreas Thaler

Andreas Thaler

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No, this doesn't change the input impedance of the device. Contact resistance is a different thing from input impedance. There's no sign of a poor contact in the scope plots of @titrisol.

To clarify, it evidently doesn't hurt to clean the connector. But it's not going to solve this particular problem.

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?
 

titrisol

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@titrisol that's pretty pronounced 'ringing' as it's often called in electronics. I'm not familiar with the innards of the Photoplug, but the first thing that comes to mind is that it matters a lot what the input impedance of the phone/tablet is. This is of little help to you, I imagine, but from a practical perspective, you could try plugging it into a different type of device and see if you get the same kind of results. My expectation is that a lower input impedance device will give better performance. This means that the device may work better on a line input than a mic input.

Can you explain the ringing part?
My photoplug has always been this way
Its a standard 3.5mm mic input in an old iPhone - should I try the dongle for lighting cable?
I cleaned both the phone and the plug but no change
 
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ic-racer

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It is not too hard to make a focal plane shutter of known velocity and slit width with which to test or calibrate a shutter tester.

This calibrated, rotating, focal plane shutter produces waveforms for 1/250, 1/500 and 1/1000 of a second. These 'standard' waveforms can be compared to the waveforms of test cameras with focal plane shutters.
DSC_0060.JPG
 

Chan Tran

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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?

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.
 

dxqcanada

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I found the photoplug too kludgey ... it took it and rigged it up with a really cheap digital storage oscilloscope ... so better.
 
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Andreas Thaler

Andreas Thaler

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I found the photoplug too kludgey ... it took it and rigged it up with a really cheap digital storage oscilloscope ... so better.

Excellent and helpful article thanks for sharing 👍

I own this little DSO too, and so far it has stopped me from thinking about a „big“ one because it offers what is essential to me.
 

tjwspm

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I use Photoplug with the Shutterspeed app on a Google Pixel 4a. As light source I use two bright 3V LEDs, which I operate with two AA batteries connected in series.

This is how I measure reproducibly 1/1000 s.
 

reddesert

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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.
 

tjwspm

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It is not too hard to make a focal plane shutter of known velocity and slit width with which to test or calibrate a shutter tester.

This calibrated, rotating, focal plane shutter produces waveforms for 1/250, 1/500 and 1/1000 of a second. These 'standard' waveforms can be compared to the waveforms of test cameras with focal plane shutters.
View attachment 360923

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?
 
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