 Fresnel diffraction by a circular aperture

For a circular aperture of radius $a$, paraxial plane-wave illumination and propagation distance $z$, define $N_F=a^2/(\lambda z)$. On axis, the radial Fresnel diffraction integral normalized by the unobstructed field givesFor a finite axial point source a distance $s$ before the aperture and detector a distance $r$ after it, replace $z$ by $sr/(s+r)$ in the phase. The formula assumes illumination amplitudes and obliquity factors approximately constant across the aperture. Odd and even numbers of Fresnel half-period zones give bright and dark axial extrema; large local intensity is compatible with global conservation of energy.