A technique for quantitatively characterizing the complex-valued focal wavefield of arbitrary optics is described and applied to reconstructing the coherent focused beam produced by a reflective/diffractive hard X-ray mirror. This phase-retrieval method, based on ptychography, represents an important advance in X-ray optics characterization because the information obtained and potential resolution far exceeds that accessible to methods of directly probing the focus. Ptychography will therefore be well-suited for characterizing and aligning future nanofocusing X-ray optics. © 2010 Elsevier B.V. All rights reserved.