A method and apparatus for determining the alignment of printing plate (14) mounted on an imaging drum (12), and applying an image to a printing plate while maintaining registration and alignment between the image and a reference edge (15) of the printing plate include mounting the printing plate on a plate-making machine (10) and then determining the locations of at least two points on the reference edge. The locations of two or more points are used to determine a transformation that is applied to image data to yield transformed image data (27). The transformed image data is used to image the printing plate. A method and apparatus for determining the locations of the points using backlighting avoids errors encountered in illuminating from the top.