Abstract
The paper is concerned with the effects of central obstruction of the aperture on the three-dimensional light distribution near the focus of an aberration-free optical system. Diagrams are given of the isophotes (lines of equal light intensity) in two selected special cases and are compared with the corresponding diagram for an unobstructed aperture. It appears that when the central obstruction is large the bright central nucleus of the three-dimensional image becomes longer and narrower, so that focal depth and resolving power are both increased. A central obstruction of ratio 0.25, on the other hand, is found to have practically no effect on the size and shape of the bright nucleus.