Diffraction PSF for an aberration-free circular aperture.
Greivenkamp, Field Guide to Geometrical Optics, p. 88
Airy Pattern (Greivenkamp, p. 88)
Irradiance pattern:
$$E(r) = E_0 \left[\frac{2J_1(\pi r / \lambda f\!/\!\#_W)}{\pi r / \lambda f\!/\!\#_W}\right]^2$$
where $r$ is the radial coordinate and $J_1$ is the first-order Bessel function
Airy disk diameter (to first zero):
$D_\text{Airy} = 2.44\,\lambda\,f\!/\!\#_W$
Working F/#:
$f\!/\!\#_W = (1-m)\,f\!/\!\# = (1-m)\,f/D$
For $\infty$ object: $m = 0$, so $f\!/\!\#_W = f\!/\!\#$
Rule of thumb (visible, $\lambda \approx 0.5\,\mu$m): $D_\text{Airy} \approx f\!/\!\#_W$ in µm