Third-order aberration analysis of a thin air-spaced aplanatic doublet with variable focal length composed of tunable focus lenses

A detailed analysis of the determination of the starting design parameters of a thin air-spaced aplanatic doublet composed of variable focal length lenses is performed using the third-order aberration theory. For such a doublet, it is possible to vary its focal length without changing the relative position of the doublet lenses, which is not possible in conventional doublets made from optical glass. The method is presented in several examples. It provides a good starting point for further optimization of such an optical design.