Embodiments of the present invention provide a method of providing image data for constructing an image of a region of a target object (40) comprising providing incident radiation from a radiation source at a target object (40); detecting, by at least one detector (50), a portion of radiation scattered by the target object; and providing image data via an iterative process responsive to the detected radiation (340), wherein said iterative process comprises estimating a wave front of scattered radiation at a plane of the detector, updating a portion of the wave front based on the detected radiation, leaving a portion of the estimated wave front substantially unchanged, setting a portion of the wave front, corresponding to a portion of radiation scattered by the target object and not detected by the detector, to one or more values, and providing image data based upon the updated wave front.