The efficient use of coherent X-rays is a crucial issue for ptychography at synchrotron facilities. We propose a method for optimizing the population of coherent modes for an optimal resolution. We show by a wave optical simulation that the intensity of a nearly diffraction-limited focusing X-ray beam can be described as an incoherent sum of a few orthogonal modes and that the first-mode flux significantly increases within a secondary source size by relaxing the requirement on the degree of coherence. We experimen…