Focus and depth of the field
The picture must be perfectly focused from the nose to the ears and the jaw up to the hair. This can lead to background obscureness, but it's not a problem. During a typical photographing, for the best quality of the face, the f-stop of the camera lens must be adjusted at two (or more) f-stop under the highest stop opening, so as to achieve enough depth field.
All the pictures will have satisfying depth focus so as to maintain a better analysis from 2 mm at the photographed face. It can be achieved a better picture analysis than 1 mm, if in a test photographing the millimeter signs, placed at the nose and the ears or the face of the person that looks at the camera, are visible.
If the camera lacks of an automatic focus, then the person must be placed always at the best position.