Caustics of lenses with fixed radial structure and different ellipticity. Source-plane angular coordinates are in units of the Einstein radius. The outer curve is the radial caustic, while the inner diamond is the tangential caustic. Point sources located outside the caustic are not strongly lensed. Sources that lie in the region enclosed by the two caustics produce three images, while sources inside the tangential caustic produce five images. The lens model adopted for this experiment consists of a stellar component and a dark matter halo, as described in Sect. 3.1. The two components have the same ellipticity.