  We present a parameter retrieval method which combines ptychography and
additional prior knowledge about the object. The proposed method is applied to
two applications: (1) parameter retrieval of small particles from Fourier
ptychographic dark field measurements; (2) parameter retrieval of retangule
with real-space ptychography. The influence of Poisson noise is discussed in
the second part of the paper. The Cram\'{e}r Rao Lower Bound in both two
applications is computed and Monte Carlo analysis is used to verify the
calculated lower bound. With the computation results we report the lower bound
for various noise levels and the correlation of particles in Application 1. For
Application 2 the correlation of parameters of the rectangule is discussed.
