Optimization programs generally use a merit function which is related to mean square spot size or wave aberration. Direct MTF optimization has been tried, but with limited success. One problem, which makes it almost unusable in the early stages of the design, is the fact that MTF values are bounded between 0.0 and 1.0 and oscillate significantly as a function of the constructional parameters. Another is the long calculation time needed for MTF. Approximations to the MTF have generally been used. Wavefront variance and wavefront difference variance are two of the more common ones that avoid both problems but don't directly optimize MTF. The variance is closely related to MTF for systems near the diffraction limit but poorly related for RMS wavefront errors greater than about 0.2 waves. The wavefront difference variance is, likewise, applicable only over a restricted region.1