  The modified Gerchberg-Saxton algorithm (MGSA) is one of the standard methods
for phase retrieval. In this work we apply the MGSA in the paraxial domain. For
three given physical parameters - i.e. wavelength, propagation distance and
pixel size the computational width in the Fresnel-Transform is fixed. This
width can be larger than the real dimension of the input or output images.
Consequently, it can induce a padding around the real input and output without
given amplitude (intensity) values. To solve this problem, we propose a very
simple and efficient solution and compare it to other approaches. We
demonstrate that the new modified GSA provides almost perfect results without
losing the time efficiency of the simplest method.
