Upper panels: combination g³ℒ cos ζ which is proportional to the observed flux from a disk surface element as a function of the azimuth for inclinations i = 30° (a), 60° (b), and 80° (c) and different emission radii r = 3 (black solid line), 5 (red dashed), 15 (green dot-dashed), and 50 (blue dotted). Lower panels: relative error (in %) on that combination computed using analytical formulae (35) and (36) for the light bending angle and lensing factor as compared to the exact calculations.