Airy point-spread function and Strehl ratio

What you are seeing: the Airy PSF for a circular aperture; the first null at $1.22 \lambda / D$. With rms wavefront error $\sigma$, the Strehl ratio drops as $S = e^{-(2\pi\sigma)^2}$ (Maréchal). The standard "diffraction-limited" criterion is $S \gt 0.8$.