It uses external interrupt and Timer1 on Arduino Uno and LCD keypad shield.
Each time the shutter has triggered, a measurement(by default up to 50 measurements) is made and can be previewed using UP/DOWN keys on the shield.
By pressing the RIGHT key, the average of the measurements is calculated and is useful for calibrating a specific shutter speed.
The SELECT key is used to show the deviation of the speeds from the standard 1-stop increments shutter speeds.
The LEFT key is used to go back to the normal mode.
An additional push button on Arduino's pin1 is used to zero the measurements.
It uses Erriez LCD Keypad Shield library: