  We investigate the problem of 3D complex field reconstruction corresponding
to unstained red blood cells (RBCs) with a single defocused off-axis digital
hologram. We employ recently introduced mean gradient descent (MGD)
optimization framework, to solve the 3D recovery problem. While investigating
volume recovery problem for a continuous phase object like RBC, we came across
an interesting feature of the back-propagated field that it does not show clear
focusing effect. Therefore the sparsity enforcement within the iterative
optimization framework given the single hologram data cannot effectively
restrict the true object volume. For phase objects, it is known that the
amplitude contrast of the back-propagated object field at the focus plane is
minimum and it increases at the defocus planes. We therefore use this
information available in the detector field data to device weights as a
function of inverse of amplitude contrast. This weight function is employed in
the iterative steps of the optimization algorithm to assist the object volume
localization. The experimental illustrations of 3D volume reconstruction of the
healthy as well as the malaria infected RBCs are presented. The proposed
methodology is simple to implement experimentally and provides an approximate
tomographic solution which is axially restricted and is consistent with the
object field data.
