Illustration of the mass‐disk degeneracy, showing the surface density (bottom) and the arrival time (top) for three circular lenses. The units, except for κ, are arbitrary. The arrival time indicates a saddle point (looking like a local minimum in this cut), a maximum, and a minimum. The dashed lines correspond to a nonsingular isothermal lens. Stretching the timescale amounts to making the lens profile steeper (dotted lines), and shrinking the timescale amounts to making the lens profile shallower (solid lines). Note that there is a limit to stretching, because otherwise κ will become negative somewhere in the region with the images—negative κ outside that region can always be avoided by adding an external monopole—but there is no limit to shrinking.