Accuracy of the approximation stated in Eq. (18) for the variables used in Wagner (2017) close to a symmetric cusp configuration of a SIE lens (left; here A and B are interchanged), close to a fold configuration of a SIE lens (centre), and close to thefold configuration of an NFW profile (right). Distances between the images |δ _AB | are determined as observed on the sky, i.e.rescaled to r_s∕D_d with the angular diameter distance from the observer to the lens D_d. Black solid lines denote the accuracy for a SIE lens with f = 0.9 or an NFW with M = 1.0 × 10¹⁵M_⊙, and red dashed lines for a SIE lens with f = 0.8 or an NFW with M = 0.5 × 10¹⁵ M_⊙. (Notation for the models as defined in Kormann et al. 1994 and Bartelmann 1996.)