I made a short video about the App:
In the video I exemplary measure the 1/60, 1/125 and 1/250 of my Rolleiflex 2,8f. Afterwards I compare the results of the app with an exact optical measurement.
Maybe it is more comprehensible now for some of you how to use this app.
Wow I noticed that your Rolleiflex has the exact shutter speed of 1/64 and 1/127. The perfect shutter speed should be 1/64 and 1/128. I am amazed at how accurate your camera shutter speeds are.
Measured (without the IPhing) is taken the first time from peak to peak, the second time from baseline to peak, the third time from an arbitrary point to peak. In the third case a "peak to peak" stretch is not easy to identify.
You seem to always measure "Actual" from baseline to peak.
From what I see, "actual" can be reliable, "measured" (with iPhone) can give any result you want, depending on where you place the beginning of the exposure, which judging from the video seems quite arbitrary.
You have to distinguish between the measurement with the iPhone-App and the optical measurement on the computer. Those are two different types of measuring and therefore you have to set the points from where you measure differently (thats why I measure the actual value from baseline to peak).
In the second measurement, there were two peaks close together. In this case, I just took a point between those two peaks. Of course, this is a measurement error, but this error is quite small, just test it out yourself !
In the third measurement the peaks aren't that distinct, but you can still see them. Just search for the peaks that have the greatest amplitude. That's not arbitrary.
Why taking a point between two peaks? Isn't that arbitrary? Can exposure begin between two peaks? Logic would dictate, IMO, that exposure begins either at the first peak, or at the second, if we have to infer from noise.
Originally Posted by Echolot
Besides, considering that both methods (your app and the Audacity method) use the noise produced by the shutter, why is the method of calculation different*?
With audacity you seem to always use "baseline to last peak". With you app you use "peak to peak" first, then "point between peaks to peak" second, then I don't remember the third, but it is not coherent with the other two.
A clear algorithm should be given, such as "baseline to last peak" every time.
This site uses "from baseline to beginning of peak excluded" that might be clearer and give more repeatable results.
And I know that you do and I criticise, and criticizing is easier than doing , I just wanted to say that your video doesn't seem to demonstrate a lot regarding the application. Which doesn't mean your application isn't useful or precise, as it can still be both things.
* Mmmh maybe the audacity method generates an electric noise through light, which is recorded by the sound card of the computer? A proper comparison would better use a proper optical measurement.
Last edited by Diapositivo; 10-07-2012 at 01:03 PM. Click to view previous post history.
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Ok I think you misunderstood something
My iPhone App is a acoustic measurement. But the measurement via Audacity, seen in the video, is a "real" optical measurement !! I use a IR-LED and an IR-Fototransistor, which is connected to the audio-input of my computer. When light hits the transistor, I can see this on the audacity signal. This method of measuring is very exact, so it can be used as a reference to the acoustic measurement from my App.
I thought this was clear to see in the video, apparently it isn't...
Well OK, let's take the Audacity measurement as "exact", I have a problem with what I would do, as a user of the App, to measure the shutter speeds if I don't have a clear method (such as: first peak to last peak, or base of first peak to base of last peak, etc.) and if I have to choose the measure extremes on the graph based on "circumstances".
Maybe, as the other site does, "base of first peak" to "base of last peak" should be easier to identify. It should always be the same method in any case or the user would end up just choosing the method that more easily matches the desired speed...
Ok now I understand your problem. Well, the problem with the acoustic measurement is, that there are no hard and fast rules how to do it (at least with the fast speeds). That's because every camera is different and therefore every signal is camera specific. You just have to experiment a bit. Look, which parts of the signal change when you compare different speeds, and which parts stay the same.
Maybe it wasn't the best idea of me to measure between those two peaks as it confuses people. But the point is, it doesn't matter that much whether you measure on the left peak, the right peak, or in between. This is the measurement error, that you have to deal with when using this app. But this error is small enough to still get usable results, and thats the point ! It may not be 100% exact, but it is not arbitrary, because the results you end up with always are in a certain range, which is about 1/3 of a f-stop.
Shutter-Speed-Tester for your iPhone
I could not see exactly how to use this at first but on seeing the video and the clarifications given here I can now see how this works. I tested out my Rolleiflex 3.5E Planar (recently back from a full CLA from Harry Fleenor) and the shutter speeds tested out very well. All within 1/3rd a stop.
Shutter-Speed-Tester for your iPhone
I've tried the app on two cameras, so far. The first I did not expect to work, because the Mercury II has a rotary shutter, and the spacing between the blades determines the timing. Rotational speed is always the same.
The second is a Ricoh XR-S. I'm not sure how best to read the results. I've attached a screen capture to demonstrate. I am not sure where to read the pulse. The test was at 1/500 sec., so I have placed the cursors at what should be that width.