We analyse errors associated with ptychographical phase determination
of coherent convergent beam electron diffraction (CBED) patterns. The
basis of this method is to observe the CBED patterns, as the probe is
scanned across the specimen, in the regions where diffracted orders ov
erlap, and measure the phases of the resulting sinusoidal intensity va
riations in such regions. The method is shown to be insensitive to the
objective lens transfer function (i.e. to defocus, astigmatism and al
ignment), the degree of spatial coherence, specimen drift, and errors
in probe scan rate. The robustness of this technique is confirmed by p
resenting results of its application to data recorded from an optical
analogue of a scanning transmission electron microscope.