Some thoughts on scalar diffraction theory

Lens designers are required to design the nearly aberration-free optics in order to meet the extremely high specification of the system. Besides the wavefront aberration generated by the lens, we should understand that an image degradation is introduced not only by the lens but also on the object or image plane. This means that the aberration-free image is not the ideal image. This discrepancy is described considering the vector nature of the light.