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The contrast transfer function of direct ptychography methods such as thesingle side band (SSB) method are single signed, yet these methods stillsometimes exhibit contrast reversals, most often where the projected potentialsare strong. In thicker samples central focusing often provides the bestptychographic contrast as this leads to defocus variations within the samplecanceling out. However focusing away from the entrance surface is oftenundesirable as this degrades the annular dark field (ADF) signal. Here wediscuss how phase wrap asymptotes in the frequency response of SSB ptychographygive rise to contrast reversals, without the need for dynamical scattering, andhow these can be counteracted by manipulating the phases such that theasymptotes are either shifted to higher frequencies or damped via amplitudemodulation. This is what enables post collection defocus correction of contrastreversals. However, the phase offset method of counteracting contrast reversalswe introduce here is generally found to be superior to post collectionapplication of defocus, with greater reliability and generally strongercontrast. Importantly, the phase offset method also works for thin and thicksamples where central focusing does not. Finally, the independence of themethod from focus is useful for optical sectioning involving ptychography,improving interpretability by better disentangling the effects of strongpotentials and focus.

Phase offset method of ptychographic contrast reversal correction