we all beholding the glory of the Lord with open face, are transformed into the same image from glory to glory, as by the Spirit of the Lord (2 Corinthians 3:18)
The Western art, especially starting with Renaissance, employs linear perspective. The picture records what our eye would see if there was an actual object coming through the picture as if it were a window. Our eye and more broadly our person becomes the central phenomenon in any modern picture.
Byzantine sacred art subtly employed inverse perspective. It cannot be explain through a simple mathematical formula. The idea is to make the object depicted in the image the center; the viewer is psychologically compelled to walk around the object to see it from several sides. This must not be overdone, so the iconographers mostly used this technique in depicting the furniture and the architecture; rarely, and very subtly on the face of Christ or the saint. It was done just enough to remind the viewer that the icon is the center and the viewer is accidental; the viewer is to be transformed to the glory of the image, not to consume the image with his eye.
All-Holy Mother of God Queen of All
Observe the clearly visible reverse perspective on the throne; the two faces are gently turned to each other suggesting that the viewer walks around and not in one spot.
Savior Not Made By Hands
Similar effect on the face: it slightly resembles two semi-profiles turned toward each other.