Optimization and synthesis of electrostatic unipotential and retarding two‐interval spline lenses have been carried out from the point of view of the spherical and chromatic aberrations. The figures of merit to express the lens performance are the spherical and chromatic aberration coefficients referred to the object, calculated in infinite magnification mode, and related to the object side focal length. The computations are based on a direct numerical ray tracing method that is suitable to solve either relativistic or nonrelativistic problems. Conditions for the calculations and optimization constraints are prescribed in compliance with the practice. A three‐electrode unipotential spline lens and a two‐electrode immersion one have been found as optimum according to the spherical aberration, and they are presented as examples. The spherical figures of merit of the unipotential and the immersion lens areCso/fo=9.17 andCso/fo=6.70, respectively. The immersion lens has a local minimum chromatic figure of merit too.