A method and apparatus are disclosed for providing image data. The method includes the steps of providing image data for constructing an image of a region of a target object, comprising the steps of: providing incident radiation from a radiation source at a target object and, via at least one detector, detecting an intensity of radiation scattered by the target object; estimating an object function indicating at least one characteristic of a region of the target object; estimating a probe function indicating at least one characteristic of the incident radiation; providing image data via an iterative process responsive to the detected intensity of radiation scattered by the target object, wherein in said iterative process each of the object function and probe function are iteratively re-estimated step by step with a running estimate of the probe function being utilised to determine a running estimate of the object function associated with the target object.