  Ptychography provides high dose efficiency images that can reveal light
elements next to heavy atoms. However, despite ptychography having an otherwise
single signed contrast transfer function, contrast reversals can occur when the
projected potential becomes strong for both direct and iterative inversion
ptychography methods. It has recently been shown that these reversals can often
be counteracted in direct ptychography methods by adapting the focus. Here we
provide an explanation of why the best contrast is often found with the probe
focused to the middle of the sample. The phase contribution due to defocus at
each sample slice above and below the central plane in this configuration
effectively cancels out, which can prevent contrast reversals when dynamical
scattering effects are not overly strong. In addition we show that the
convergence angle can be an important consideration for removal of contrast
reversals in relatively thin samples.
