The mathML formulae

The width of the field the camera captures is relative to how far away any object is, more precisely:

which gives us:

So what's the view width at the point where the face is? We know that the proportion of the screen the face will fill is given by:

However, this is the same for pixels, so:

And this gives us:

Inserting this, we get:

In our case ${\mathrm{viewwidth}}_{\mathrm{px}}$ is the width of the canvas, ${\mathrm{facewidth}}_{\mathrm{px}}$ is the width of the face on the canvas, and we assume ${\mathrm{facewidth}}_{\mathrm{cm}}$ is around 17 cm.