The efficient use of coherentX-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 experimentally demonstrate it by means of multiple-mode ptychography with a synchrotron X-ray and achieve the high-resolution imaging of a weak-phase object. The present approach enables the high-resolution and high-throughput observation of weak-phase objects in materials science and biology.