A method of determining the location of an optical feature in an image projection system, the method comprising: 1) projecting a first image of the feature onto a detector with a lens in a first position; 2) sensing, with the detector, the position of the first image of the feature; 3) projecting a second image of the feature onto a detector with a lens in a second position laterally spaced from the first position; 4) sensing, with the detector, the position of the second image of the feature; and 5) deducing the location of the defect from the difference between the positions sensed in steps 2) and 4).