WILLIAMSRESTMTF Utility function for WILLIAMSMTF
mtf = WILLIAMSRESTMTF(s)
Compute the portion of the MTF measured by Williams et. al. that is not
accounted for by diffraction.
Williams, D.R. Brainard, D.H., McMahon, M., and Navarro, R. (1994).
Double pass and interferometric measures of the optical quality of the
human eye. Journal of the Optical Society of America A, 11, 3123-3135.
Formulae given in Equation 1 ff.
Spatial frequency passed in cycles/deg.
See also WILLIAMSMTF