Embodiments of the present invention provide a method of providing image data for constructing an image of a region of a target object, comprising providing incident radiation from a radiation source at a target object, detecting, by at least one detector, a portion of radiation scattered by the target object with the incident radiation or an aperture at first and second positions, and providing image data via an iterative process responsive to the detected radiation, wherein in said iterative process image data is provided corresponding to a portion of radiation scattered by the target object and not detected by the detector.