A method of retrieving a phase of a wavefield comprising the steps of: providing an estimate of the wavefield φ0 at an initial plane; and propagating the wavefield to and fro between an entrance plane being a plane having an area to which the wavefield is confined and a detector plane via a wavefield transform device, wherein at the entrance plane a support constraint is applied and at the detector plane a magnitude constraint is applied, the wavefield transform device being arranged to apply a wavefield transform function to the wavefield, wherein the wavefield transform function is characterised by a finite deviation from a lens function.