Schematic lensing geometry related to the determination of the magnification factor μ. Left: sketch illustrating the angles and distances introduced in the main text. Right: first three rays of orders 1, −1, 2 connecting the source with a small angular separation from the caustic line ψ to the distant observer. Top right: source is located behind the lens (see Sect. 3.1). Bottom right: source is located in front of the lens (see Sect. 3.2).