Comparison between the approximations of the critical curve as a parabola (cusp) or a line (fold) for a given source position a) shows that the fold critical point always lies below the cusp critical point in the image plane d). Depending on the distance of the source to the caustic a)–c) and the images to the critical curve d)–f), the fold approximation is more precise than the cusp approximation b) and e) or vice versa c) and f). Image C, not being relevant for the argument, is omitted here.