 Solution

In Fraunhofer diffraction, the source and observation geometry make the incident and outgoing wavefronts effectively planar across the aperture. For direct propagation over distance $z$, the quadratic variation in propagation phase across a slit of width $w$ must be negligible, approximately $w^2/(\lambda z)\ll 1$. The far-field angular amplitude is then the Fourier transform of the aperture's complex illumination. A converging lens also produces this angular distribution in its rear focal plane for collimated illumination, without requiring a physically enormous observation distance. The approximation concerns the aperture-dependent phase, not the absence of diffraction.