Here some groupings are used: Γ Γ is a constant that gives the relative size of the smallest aperture of the zone plate compared with the average wavelength of the beam and Φ Φ ′ is the modified beam width, which is used through the derivation to avoid explicitly operating with the constant airy term: Φ Φ ′ 2 = σ σ c m 2 + ( M r S 3 ) 2 .