The location of caustics β_caustic as a function of the lens amplitude A. For both the Gaussian and top-hat models, the dimensionless locations β of the outer caustics (dotted line) increase linearly with the lens amplitude A, whereas those of the inner caustics (dashed line) increase with A only very slightly, and thus they remain on the order of unity for a large range of A. As a consequence, the ratio between the outer and inner caustics (solid line) increases linearly with the lens amplitude A, suggesting a much greater range of the three-image zone for a lens with a greater amplitude. The point mass model is not shown because of its lack of outer caustic.