Calculate the distance between your smartphone screen and your face using the front facing camera. The idea is pretty simple and best explained in an image.

An object or a "distance" will seem smaller once its further away from the camera than when its closer to it. So one should be able to calculate the distance between camera and face while using the eye distance at a reference length, in our case 29.7cm (the length of an A4 paper), and multiplying the reference length with the change of eye distance.

Close: Far:

For a video
https://www.youtube.com/watch?v=-6_pGkPKAL4

And the link to a more thorough analysation
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6825217
If you do not have free access to ieee send me a message and I will give send you a copy