Well, I´ve freehand modeled heads in the past (with fotoreference ofc) but what I´ve always found to be the hardest thing to overcome is the perspective distortion from a picture taken. you could minimize that perspective distortion by going back as far as possible and zoom in with the camera to the max, but still - getting good resolution pictures with minimal perspective in it (orthographic compact cameras would be great..) isn´t the easiest.

So I was looking for a cheap method to scan the actual real-life geometry. Everyone knows 3D-Scanning can become quite expensive but after some googling I found a small company which scannes your face in 3D and then laseres into a high quality glass cube threedimensionally. Their product sold is this lasered cube and it costs you 20-200 Euros depending of the cube size (2-20 cm). Usually they don´t sell Data and no one asked before I did. So they found it to be interesting. Since it was Valentine´s Day My girl and me got us one of these cubes. The other 10 Scans I let them take from her head were allmost for free. (cost me around 20 bucks). I found this to be the cheapest way to get the most precise reference for starting the modeling process.

Here´s a sample of the lasered glass cube.