  We address the problem of reconstructing phase-shifting objects from their
single shot in-line holograms. We show that a phase-shifting object cannot be
reliably recovered from its in-line hologram by non-iterative reconstruction
routines, and that an iterative reconstruction should be applied. We
demonstrate examples of simulated in-line holograms of objects with the
following properties: purely absorbing, both absorbing and phase shifting, and
strong phase-shifting. We investigate the effects of noise and interference
contrast in holograms on the reconstruction results and discuss details of an
optimal iterative procedure to quantitatively recover the correct absorbing and
phase-shifting properties of the object. We also review previously published
reconstructions of experimental holograms and summarize the optimal parameters
for retrieval of phase-shifting objects from their in-line holograms.
