Within geometric raytrace optics, a perfect optical system is stigmatic, meaning each object point focuses to a single image point.
Wavefronts diverging from an object point are always spherical.
After being transformed by a stigmatic optical system, converging wavefronts are again spherical, but are now centered around the image point. An aberration is any departure from sphericity or shift in centration of a converging wavefront leading to blurred or systemically moved image points. Petzval curvature , distortion . Optics any aberration measure extending in the longitudinal or axial direction .