More over at Stephen Jones’ blog on the Vignon markings
[A]s can be seen above, the Ravenna Pantocrator mosaic has at least thirteen of the fifteen Vignon markings on the Shroud [see part #2 (1)] namely: "(2) three-sided `square’ between brows, (3) V shape at bridge of nose, (4) second V within marking 2, (5) raised right eyebrow, (6) accentuated left cheek, (7) accentuated right cheek, (8) enlarged left nostril, (9) accentuated line between nose and upper lip, (10) heavy line under lower lip, (11) hairless area between lower lip and beard, … (13) transverse line across throat, (14) heavily accentuated owlish eyes, (15) two strands of hair" 
I do find the Vignon markings very telling, when considered collectively, in large numbers. But we must be careful when considering them independently or just a few at a time. Jones is sensitive to that.
For instance, what are we to make of the “three-sided `square’ between brows,” sometimes referred to as a squared off U or a topless box? Too much, sometimes. It could be a defect in the cloth that was seen by an artist as a facial feature. It could be that it really was a feature of Jesus’ face. Or it could be, as some have suggested, an object resting on the face, a phylactery perhaps.
I recall a discussion when someone said it must be a defect of the cloth because Jesus was too young to have such an old-man wrinkle. I have a three-sided square in exactly the same place, but I’m old. So I asked my young thirtyish Jewish neighbor to furl his brow, just as I showed him I could do, to see if a young Jesus could have this feature. It didn’t work. However, another neighbor who is half Welsh and half Italian and is only twenty-seven years old can make a perfect topless box above his nose by squinting just slightly in bright sunlight. If I trust only some of the Vignon markings, then since Leonardo da Vinci was at least half Italian, I must conclude that the shroud is a medieval photograph of him taken in bright sunlight.
I did try to find a picture of Leonardo da Vinci with the feature. There weren’t all that many pictures of the old man. But I did find pictures of another Leonardo with a topless box above his nose. And he is a young fellow. And Italian. This should cause Picknett and Prince to rethink their theory. It cannot have been the great medieval photographer, Leonardo; no topless box on him.
I do think the Vignon markings may tell us something. We must exercise care, however.