Strong lensing cross-section of an extended source: large source size regime (θ_e, s ≳ θ_Ein). The cross-section is plotted as a function of the ratio between the Einstein radius of the lens and the half-light radius of the source. The source model is fixed to a circular Sérsic profile with n = 1, detected out to the half-light radius. The source half-light radius measures 20 pixels, and the PSF has an FWHM of two pixels. The lens model is varied: starting from the model of Fig. 9, the critical surface mass density is increased to produce progressively smaller values of θ_Ein. The solid line shows the lensing cross-section, in units of the footprint area of the source in the absence of lensing, as a function of the ratio between the lens Einstein radius and the source half-light radius. The dashed line delimits the area enclosed within the lens caustics, and the dotted line shows the value of the cross-section corresponding to the area of the source. The dash-dotted line is the FWHM of the PSF.