This paper considers the anomalous propagation of phase through the focus of an optical system. We observed the interference rings produced by two collinear spherical waves coming from the same laser source with different focal points. Then three regions are determined along the optical axis: R1, befor focuses; R2 between focuses; and R3, after focuses. In each of them we pictured interference rings which are complementary in intensity with a CCD camera. The complemnetary relationship is caused by the sudden change of phase by π through each focus.