Optical EngineeringPetzval sum in triplet design: trend or optimum value?
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Based on the analysis of known, corrected, classical triplet type objectives and given the basic specification requirements, diagrams of the constructional length and image distance of their equivalent thin lens systems are prepared. By keeping the constructional length and image distance constant and applying simple formulas, the focal lengths and air gaps of the thin lens triplet variations can be generated. These variations can be used for selection based on any criteria, such as, for example, the Petzval sum. According to the original description of the Petzval sum, to our computational experiences and to the examples given, which were the results of precise corrections, we concluded that also in case of triplets, the designer should target the minimal Petzval sum, which is constrained only by the basic specification data.