Camera: Measure Distance to Face

I want to create a tool to measure the distance from the front-facing camera (towards the user looking at the screen), to the user's face. Please answer: iPhone or Android? (I listed both in this project, but obviously this is just for one or the other) How would you do it? What do you foresee the shortcomings, ie. accuracy, need for calibration, complexity, compatibility (in Android's case)?