WESTLSFMINUTES Compute Westheimer estimate of human LSF
lsf = WestLSFMinutes(distance)
Compute Westheimer’s LSF function as a function
of passed distance. Distance passed in minutes of arc.
Formula from Westheimer G. 1986. The eye as an optical instrument. In
Handbook of perception and human performance, KR Boff, L Kaufman,
JP Thomas (eds). New York: Wiley. Equation 8.