A method and apparatus for sharpening gastrointestinal (GI) images are disclosed. A target distance between the target region and the imaging apparatus is determined for a target region in the regular image. One or more filter parameters of a de-blurring filter are selected from stored filter parameters according to the target distance. A processed target region is generated by applying the de-blurring filter to the target region to improve sharpness of the target region. A method for characterizing an imaging apparatus is also disclosed. The imaging apparatus is placed under a controlled environment. Test pictures for one or more test patterns are captured at multiple test distances in a range including a focus distance using the imaging apparatus. One or more parameters associated a target point spread function are determined from each test picture for characterizing image formation of the imaging apparatus at the selected distance.