Ptychography provides high dose efficiency images that can reveal light elements next to heavy atoms, as well as the ability perform post collection aberration correction and superresolution. However 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 be counteracted in direct methods by using defocus, even if applied post collection. Here we explain why intermediatethickness samples show the best contrast with the probe focused to the middle of the sample and more generally explain the contrast reversals and their counteraction in terms of phase wrap in reciprocal space. Multiple scattering is not required to produce phase wrap, as it can occur for even a single atom of very high atomic number, but it does effect the optimal defocus.