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Abstract
Images of circular openings in opaque screens have been studied to investigate the effects of the mode of illumination. For this purpose the concept of effective source has been explicitly employed. It is shown that the image structure depends very significantly on the mode of illumination, and at least with circular openings, equality of the numerical aperture of the illuminating condenser with that of the imaging objective does not ensure a state when effectively incoherent illumination may be assumed. This affects, for example, the measurement on the sizing of particles from the distribution of intensity in the image formed with a microscope set-up. Moreover, in some cases the intensity distribution shows a marked dip at the centre, which may again lead to a confusion in the interpretation of the image.