We studied the diffraction phenomenon of a circular aperture that was illuminated by a monochromatic converging spherical wave. The theoretical part of this study was based on the scalar Huygens–Fresnel integral. In our experiments we used a circular detector centered on-axis to measure the encircled energy. We varied both the radius and the longitudinal position of the detector. The principal maximum of the encircled energy received by the detector was usually not in the geometrical focal plane, and its position changed abruptly when the radius of the detector was varied. These abrupt changes look like sawtooth variations, which are superimposed upon a slowly changing curve. Our experiments were in a good agreement with calculations.
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