Use of iPhone apps for exposimeter calculations

I just discovered a good app on the AppStore in my IPhone 6 plus that works wonderfully like exposimeter, and it is iLightMeter. Just costs a very little and works perfectly in managing light, times, ISO and f. It is useful for shoting in A or M mode.

Will try to use it instead of buying expensive instruments, to learn how it really works for all.

I just used it for this photo:

Photo credits: Debora Teresa Gattuso.