Fig. 1

a Model of a wedge-shaped crystal whose columns contain a different number of atom. The exit surface of each column differs in z-height (beam direction) with respect to a common image plane. b Representation (Argand plot) of the complex pixel values at the centers of a column. Blue dots move along the black circle as the number of atoms in a column increases; red dots move along the red, dash line as the propagation distance Δf changes from 1 to 3. c The propagation intensity for a set of experimental gold columns [3]. d The “true” focus value (blue) is detected if the propagation intensity reaches a maximum