The quality of images of an electron optical system is dominated by the performance of an objective lens. This is because the angle spread of an electron beam becomes largest in an object plane. In electron spectroscopy with imaging, image intensity often has priority over spatial resolution, and then the image quality is determined by the spherical aberration of an objective lens. In this ... [Show full abstract] chapter, the imaging process for incoherent illumination under the influence of spherical aberration is discussed. Evaluation of such optical systems using the theory of spatial-frequency response is also described.