1. An iterative method of processing a pixellated image comprising image pixels to retrieve a phase distribution representative of the image in the Fourier domain, the method comprising: padding the image pixels with non-image pixels to increase the total number of pixels in the pixellated image; associating each pixel of the pixellated image with a phase value to form a first data set having plural elements, each element having amplitude information and phase information; and (n+1) iterations of the following steps: performing a Fourier transform of the first data set to produce a second data set having plural elements, each element having amplitude and phase information; quantising phase information from the second data set; forming a third data set having plural elements, each element having amplitude and phase information, the phase information being derived from the second data set and; inverse Fourier transforming the third data set to produce a fourth data set having plural elements, each element having amplitude and phase information; overwriting the amplitude and/or phase information of the fourth data set to produce a fifth data set having plural elements, each element having amplitude and phase information, wherein overwriting comprises processing the non-image pixels differently to the image pixels and attenuating the amplitude information of each non-image pixel of the fifth data by an attenuation factor and/or processing the amplitude information of the each image pixel utilising a gain factor, wherein the attenuation factor and/or gain factor are variable and dependent on the iteration number of the iterative method; and using the modified fifth data set as the first data set of the next iteration.