1 The little one
One clear, straight-on photo, looking at the camera with a relaxed face. Both eyes must be visible — no sunglasses, no hat brim across the eyes — because everything is measured relative to the gap between the pupils. Big in the frame beats far away, and a neutral expression beats a big grin, because a grin genuinely moves the mouth.
2 Who to compare against
Mum, Dad, a grandparent, an uncle — up to six. Same rules: straight on, both eyes visible, relaxed face.
3 Compare
Each reading measures the same faces from a slightly different crop. Finding a face is never pixel-perfect, so anything that shifts between readings is measurement noise — and where that noise is bigger than the difference between people, the result says so instead of picking a winner.