Measurements of the photon ring shape carried out over insufficiently long baseline windows display characteristic transition effects. In this example, the BH spin is a = 0.5, the observer inclination is i = 45°, and the emission profile (Eq. (50)) has parameters μ = 3r₊/2, γ = −1, and σ = 0.5M. (A) Projected diameters d_φ inferred from the periodicity of the visamp in the range [550, 650] Gλ via the multi-fit method (Sect. 3). With this baseline choice, d_φ does not follow the circlipse shape (Eq. (28)) because it only tracks within the blue zone, which is separated by green transition zones from the gray zone where it corresponds to . The visamp profile |V(u, φ)| over this baseline window (shown as an orange band) is dominated by the n = 2 ring at φ = 60° in the blue zone (B), by the n = 1 ring at φ = 140° in the gray zone (D), and by neither at φ = 100° (C).