= 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 $w^2/(\lambda z)\ll1$. 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>.
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