  We report on a ptychographical coherent diffractive imaging experiment in the
water window with focused soft X-rays at $500~\mathrm{eV}$. An X-ray beam with
high degree of coherence was selected for ptychography at the P04 beamline of
the PETRA III synchrotron radiation source. We measured the beam coherence with
the newly developed non-redundant array method. A pinhole $2.6~\mathrm{\mu m}$
in size selected the coherent part of the beam and was used for ptychographic
measurements of a lithographically manufactured test sample and fossil diatom.
The achieved resolution was $53~\mathrm{nm}$ for the test sample and only
limited by the size of the detector. The diatom was imaged at a resolution
better than $90~\mathrm{nm}$.
