If an object under a phase contrast microscope is illuminated with light coming from a point source in such a manner that the image of the source is projected upon one of the edges of the phase annulus, the image of the microscopic object shows certain peculiarities which cannot be accounted for under the hypothesis that the point source is imaged upon the annulus without aberrations. Experiment indicates that in cases where the edge effect is observed, the image of the light source is defocused with respect to the phase annulus. The theory of the edge effect may be adapted from the diffraction theory of the Foucault knife-edge test of Gascoigne and Linfoot.
L. IVAN EPSTEIN, "The Edge Effect in Phase Contrast," J. Opt. Soc. Am. 41, 134-136 (1951)