Airy disk: I(r) = [2J₁(πDr/λf) / (πDr/λf)]² where J₁ is the Bessel function of first kind.
Rayleigh criterion: two point sources are just resolved when the central maximum of one falls on the first minimum of the other.
First zero at r = 1.22λf/D → angular resolution θ = 1.22λ/D.