  The contrast transfer function of direct ptychography methods such as the
single side band (SSB) method are single signed, yet these methods still
sometimes exhibit contrast reversals, most often where the projected potentials
are strong. In thicker samples central focusing often provides the best
ptychographic contrast as this leads to defocus variations within the sample
canceling out. However focusing away from the entrance surface is often
undesirable as this degrades the annular dark field (ADF) signal. Here we
discuss how phase wrap asymptotes in the frequency response of SSB ptychography
give rise to contrast reversals, without the need for dynamical scattering, and
how these can be counteracted by manipulating the phases such that the
asymptotes are either shifted to higher frequencies or damped via amplitude
modulation. This is what enables post collection defocus correction of contrast
reversals. However, the phase offset method of counteracting contrast reversals
we introduce here is generally found to be superior to post collection
application of defocus, with greater reliability and generally stronger
contrast. Importantly, the phase offset method also works for thin and thick
samples where central focusing does not. Finally, the independence of the
method from focus is useful for optical sectioning involving ptychography,
improving interpretability by better disentangling the effects of strong
potentials and focus.
