Plots for ρ = 0.03 and u₁ = 0.6. Upper panels: Shown in the plot on the left are the lens (black dot in the centre), the (unseen) extended source (orange disk), and its two lensed images (red and blue arc-shaped images), displayed in (x_E, y_E) coordinates which are normalised by the angular Einstein ring radius θ_E. The plot on the right displays a three-dimensional view of the squared visibility |V_E|² in the Einstein u_Eυ_E-plane, normalised by . Middle panels: Shown in the plot on the left is a contour plot of |V_E|², while the plot on the right shows the phase of the complex visibility ϕ = arg V_E (the thin white line is a visualisation artefact when ϕ jumps from −π to π, or vice versa). Lower panels: Difference in squared visibility Δ|V_E|² between either the point-source (left) or the thin-arcs (right) approximation and the exact calculation. The colours saturate for a difference of ±0.1. The inner dashed circle marks the typical angular resolution (radius 0.25), and the outer circle (radius 0.5) twice the typical resolution.