Upper panel : Radial profile of the Poisson spot created by an opaque circular disk of radius R is plotted in red, while the Airy disk profile created by a lens of same radius is plotted in blue. Both profiles are normalized to unity at r = 0. The Poisson spot is described by the function J₀²(u) (Eq. 27), while the Airy disk is described by the function [2J₁(u)/u]² (Eq. C.1). The normalized geometrical optics approximation given by Eq. 9 is plotted as a red dotted line which diverges to infinity at the origin. The Poisson spot is narrower than the corresponding Airy disk by about 60% (Eqs. 29 and C.2). The fringes around the Poisson spot damp out more slowly (∝ 1/u) that those of the Airy disk (∝ 1/u³); see Eq C.3. Middle panel : 2D-map of the Poisson spot, corresponding to the red profile in the upper panel. Lower panel : 2D-map of the Airy disk, corresponding to the blue profile in the upper panel.