Strong lensing cross-section of an extended source: small source size regime (θ_e, s < θ_Ein). The cross-section is plotted as a function of the ratio between the half-light radius of the source and the Einstein radius of the lens. The lens model is fixed to be the same as in Fig. 8, with axis ratio q = 0.7. The source is a circular exponential profile. Each solid line corresponds to a different value of the total unlensed flux of the source. The flux, f, is expressed in terms of the background noise rms fluctuation measured over an area equal to . The vertical dashed lines correspond to the maximum size for which a galaxy with a given flux can be detected in the absence of lensing. The horizontal dotted line indicates the cross-section for a very bright point source (i.e. the area enclosed within the caustics).