Unfortunately, it isn't measuring the amount of time light passes through the shutter. At slow speeds, it will be sort of accurate.High speeds, less so. On an SLR, there's no knowing what it might be measuring. Good luck, you'll need it.
I've compared results from contraptions like this to a proper test of shutter speeds using a phototransistor and a scope with calibrated time base, these give an approximation at best. Useless for my purposes, which involve measuring how long the shutter actually passes light.
Edit - There are lightmeter apps. Why not write a shutter speed measuring app. that uses the light sensitive element of the cellphone to actually measure the shutter speed, not the sounds the shutter makes, i.e. doing it properly??
Last edited by E. von Hoegh; 09-24-2012 at 12:16 PM. Click to view previous post history.