Embodiments of the invention provide a method of determining one or more characteristics of a target object, comprising determining a first phase map for at least a region of a target object based on radiation directed toward the target object, determining one or more further phase maps for a sub-region of the region of the target object, determining a number of phase wraps for the sub-region based on a plurality of phase maps for the sub-region, and determining a characteristic of the region of the target object based on the number of phase wraps for sub-region and the first phase map. Embodiments of the invention also relate to a method of determining one or more characteristics of a target object, comprising determining a phase map for at least a region of a target object based on one or more diffraction patterns, determining a wavefront at a plane of the object based upon the phase map, and determining a refractive property of the object based on the wavefront.