The file 'Double Gauss minimum MTF.zmx", which you can download from the last page of this article, shows our double Gauss file with an updated merit function. In addition to the default wavefront merit function, it contains multiple MTF operands to compute the MTF across the field of view, and then the MINN/OPGT structure discussed on the previous page.

One way to sample the MTF versus field is just to define multiple field points across the field of view in the Field dialog box (System...Field). This limits us to 12 field points however, and may not be helpful if multiple field points are already needed, or when tolerancing. A more elegant approach is to use the FDMO operand to re-define the field of view of a given field point 'on the fly':



In this case we use the FDMO operand to re-define the hy value of field point 2 from 0 to 1 in steps of 0.1, and we compute the MTFS and MTFT at 30 cycles/mm at each value of hy. The MTF* operands have zero weight, so they do not affect the optimization. Then:



Line 40 gets the minimum value of all the merit function lines between operands 7 and 38, and line 42 requires this value to be greater than 55%. Why 55%, when the customer's specification is 50%? Because we have not yet toleranced the design, and so we need to budget some extra performance to allow for the degradation that manufacturing tolernaces will cause. After optimizing with this new merit function we go from the original MTF vs field curve:



to this curve:



So we have computed the lowest value on the MTF vs field plot, and optimized this value to be >50%