Tolerance on defocus precisely locates the far field (exactly where is that far field anyway?)

The Fraunhofer criterion defines the location of the boundary between the Fresnel and the Fraunhofer diffraction regions and thus determines the location of that region commonly referred to as the far field. The Fraunhofer criterion is usually given as an axial distance much greater than some amount relative to the maximum dimension of the aperture. By recognizing that Fresnel diffraction patterns are merely defocused Fraunhofer diffraction patterns, we show that the Fraunhofer criterion can be written precisely in terms of an allowable tolerance on defocus. This new criterion provides insight that is useful to optical designers and engineers who routinely deal with such tolerances.