8 - Combinations of Achromatic Doublets

Summary

The achromatic doublet is a fundamental building block in lens design because it is corrected for chromatic aberrations, and can also be corrected for spherical aberration and coma aberration. The early lens designers explored all combinations of two achromatic doublets. This chapter discusses some of the solutions found by those designers. In doing so, insight is gained into how simple lens combinations are designed. Emphasis is given to how the primary aberrations are controlled in doublet combinations, as this knowledge is important to become skilled in lens design. Providing degrees of freedom to correct the primary aberrations is a first step toward the optimization of a lens. In practice, the primary aberrations may not be fully corrected so that higher order aberrations might be balanced against the primary aberration residuals. Once a primary aberration solution was reached in the examples given in this chapter, then they were optimized with real rays in a lens design program by minimizing RMS spot size across the field of view. Thus, a lens design method is to find a primary aberration solution and then optimize it with real ray tracing.