Illustrating the difference between the geometrical magnification μ = d²θ/d²β and the observable magnification . The causes of discrepancy are: (i) the position of a source at z may be shifted from the background to the perturbed case, ; (ii) the beam’s cross section may be tilted due to lensing, d²A_⊥ = d²A_z cos ι. This illustration equally applies if z is replaced with λ, η, ⋯.