The procedure followed in optimizing a five-element diffraction- limited laser scan lens to eliminate one of the elements is described. The starting procedure uses thicknesses found in the five-element design except for the two central positive elements, which are combined to make a single thick third element. The resulting four-element lens employs high-index glass for each element and achieves diffraction-limited performance.
Morris M. Klein,
"Optimization to create a four-element laser scan lens from a five-element design," Optical Engineering 31(6), (1 June 1992). https://doi.org/10.1117/12.56185