Abstract
Until recently corrections of the defects of the optical system in the lens design such as a triplet lens system have been based on classical conceptions of the image formation through refraction surfaces of the system, or based on the third-order Siedel's aberration coefficient. This paper continues emphasizing the use of the caustic merit function, that describes the spread of the caustic surfaces from an ideal image point. This caustic merit function is then used to optimize a triplet lens. Results show that optimizing the caustic merit function is still the fastest and an effective method in the optical design in the case of plane waves.