Evaluation of the SIE profile (fold): ratio of Eq. (4)divided by the true value for image A (black solid line) and image B (red dashed line) a). Ratio of Eq. (3)for image A and B and for both images under the symmetry assumption that their orientation is orthogonal to the critical curve (blue dotted line, green dash-dotted line) b). Slope of the critical curve, Eq. (5), at the fold using the ratios obtained in a) and b) c). The grey-shaded area marks 10% deviation from the true ratio.