The longitudinal coherence function at the Advanced Photon Source beamline 34-ID-C has been measured by a novel method and the coherence length (xi(L)) determined to be, xi(L) = 0.66 +/- 0.02 microm. Three dimensional Coherent X-ray Diffraction (CXD) patterns were measured for multiple Bragg reflections from two Zinc Oxide (ZnO) nanorods with differing aspect ratios. The visibility of fringes corresponding to the 002 crystal direction for each reflection were found to be different and used to map the coherence function of the incident radiation. Partial coherence was found to be associated with amplitude 'hot' spots in three dimensional reconstructions of the crystal structure.