it's been a while since this thread started, but I have some news regarding the App

First, there will be an update very soon (it is still in review by Apple). It will fix a major problem regarding iOS 5 users (the content was displayed in wrong display orientation) and offer the possibility of measuring and saving old shutter speeds like 1/5, 1/10, 1/50 etc. as suggested in this thread.

Second, there is now a possibility of doing real light-measurements with this App !!
As discussed in the thread, the acoustic-measurement has its limits. Much better would be a light measurement via phototransistor. Unfortunately, the iPhone doesn't has such a sensor build inside. BUT, what most people don't know: The iPhone has a microphone-input inside the earphone-socket. This offers the possibility of attaching a phototransistor to the iPhone, and do light measurements.

I did some experiments, and found a very smart solution I think:

This small plug contains this very simple schematic:

It doesn't need batteries, it just uses the small voltage which the iPhone provides (for the electred microphone). The resistor is 4,7 kOhm and the fototransisitor I use is a BPX 38. The resistor is needed so that the iPhone recognizes the plug as a microphone.

Plug it into the headphone socket, and voilą, you have a mobile light-measureing shutter tester:
Foto am 17.04.13 um 17.29.jpg

To use it, just take a camera, remove the back, light it with a light source from the back and position the plug to the lens:
Foto am 17.04.13 um 17.39 #2.jpg

Then you proceed as usual.
What you get is a very nice and clean signal with clear visible peaks:
Here with 1/60:


It works very nice, even at 1/500:

It is very simple to build a plug like this, the only thing which is a bit hard to get is the 4-pin phone-connector.
Would be great if someone would try this out !