Illustration of the isolated lens model. Galaxies are depicted by black pixels, their matter haloes as red disks. For simplicity all haloes are identical in this visualisation. Left panel: lenses are distributed randomly on the sky. Right panel: lenses cluster to produce clumps with a common matter envelope, yet still described as sums of individual haloes. On the statistical level, this clumping is quantified by non-vanishing 2nd-order and 33d-order correlation functions ω and Ω, respectively.