mtf = GoodmanDiffrac(s,s0)
Compute Equation 6-31 from page 120 of Goodman.
This is the diffraction-limited optical MTF,
computed given the coherent diffraction limit s0.
Goodman, J. W. (1968) Introduction to Fourier Optics.
San Francisco: McGraw-Hill.
Note that in the Williams et al. paper (ref??), this
formula is given for the incoherent diffraction limit,
which is twice the coherent limit.
Also see DiffractionMTF.