Strong lensing cross-section of a circular power-law lens and a point source. A lensing event is defined as the detection of an emission line within an aperture of 1.5″, and of at least two images in photometric data. Top: σ_SL of a lens with θ_E = 1.2″ as a function of the slope γ. The thick green line corresponds to a source with intrinsic emission line flux equal to one third of the flux required for a spectroscopic detection, and a broadband flux equal to that needed for a photometric detection. Different lines were obtained by rescaling both the emission line and broadband flux by the amount indicated in the legend. Bottom: geometric factor of the lensing cross-section, that is the value of σ_SL for the reference source (corresponding to the green line in the top panel), as a function of both θ_E and γ.