  Ptychography currently seems the most natural and effective method of
approaching the diffraction limit of optical resolution. Schematic diagram of a
ptychography microscope does not contain refractive or focusing elements. It
includes a source of coherent illumination, a platform for (macroscopic)
movement of the object, and a detector for recording transmitted or reflected
radiation from the object. The detector is connected to a computer that
processes diffraction patterns. In this paper, after a brief introduction to
the history and current state of ptychography, we consider in detail the wave
packet method for calculating the wave field at a detector in the far zone. It
allows to establish the relationship between the fields on the object and the
detector up to a numerical aperture of ~1. Theoretically substantiated formulas
are proposed that determine the size and discretization step of a domain on an
object, for given sizes of detector, pixel and object to detector distance.
Based on these formulas, a comparison with the paraxial approximation for a
point source is performed, as well as modeling of ptychography imaging by the
PIE algorithm.
