Sorry if this has been discussed to death already ... but I want to find a reliable & easy method to test my shutter speeds. I download an app for my iPhone that uses the widely discussed audio test, but find it completely unreliable. What am I actually looking at here? Where is the start/end of the shutter?? Example for a 1/100 s shutter speed and my best guess at the bottom image.

Is the Phochron optical tester a better way?