  X-ray near field holography has proven to be a powerful 2D and 3D imaging
technique with applications ranging from biomedical research to material
sciences. To reconstruct meaningful and quantitative images from the
measurement intensities, however, it relies on computational phase retrieval
which in many cases assumes the phase-shift and attenuation coefficient of the
sample to be proportional. Here, we demonstrate an efficient phase retrieval
algorithm that does not rely on this homogeneous-object assumption and is a
generalization of the well-established contrast-transfer-function (CTF)
approach. We then investigate its stability and present an experimental study
comparing the proposed algorithm with established methods. The algorithm shows
superior reconstruction quality compared to the established CTF-based method at
similar computational cost. Our analysis provides a deeper fundamental
understanding of the homogeneous object assumption and the proposed algorithm
will help improve the image quality for near-field holography in biomedical
applications
