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Ptychography provides high dose efficiency images that can reveal lightelements next to heavy atoms. However, despite ptychography having an otherwisesingle signed contrast transfer function, contrast reversals can occur when theprojected potential becomes strong for both direct and iterative inversionptychography methods. It has recently been shown that these reversals can oftenbe counteracted in direct ptychography methods by adapting the focus. Here weprovide an explanation of why the best contrast is often found with the probefocused to the middle of the sample. The phase contribution due to defocus ateach sample slice above and below the central plane in this configurationeffectively cancels out, which can prevent contrast reversals when dynamicalscattering effects are not overly strong. In addition we show that theconvergence angle can be an important consideration for removal of contrastreversals in relatively thin samples.

On central focusing for contrast optimization in direct electron ptychography of thick samples