Example of a visibility amplitude profile at fixed angle φ = 0° in the baseline plane, obtained from a BH image simulated with Gyoto and exhibiting the clean ringing expected from the narrow photon ring. Three regimes (see Eq. (23)) are visible: in the range [0, 20] Gλ, the direct (n = 0) image dominates; for u ∈ [20, 120] Gλ, the first (n = 1) subring prevails; beyond 120 Gλ, the n = 2 ring signature takes over.