Critical lines (black) and caustics (grey) of the elliptical power law profile lens. Also shown is an ellipse with semi-minor axis equal to the scale length b and axis ratio q (red, dotted). For the purpose of illustration, the scale length b decreases as q^1/2. The slope of the power law profile varies from t = 0.25 (left) to t = 1.75 (right) in steps of 0.5. The axis ratio varies from q = 0.8 (top) to q = 0.2 (bottom) in steps of 0.2.