An optical sensor for detecting the position of marks on a medium, wherein relative motion is provided between the optical sensor and the medium during an optical sensing operation. The optical sensor produces an electrical sensor signal, and has a field of view at the media in a direction of the relative movement. The marks have a nominal dimension in the direction of the relative movement, and the field of view is larger than the nominal dimension. This produces a sensor signal with a clear and relatively sharp peak in response to scanning the media mark. A method of sensing directional aberrations among ink-jet nozzles of a plurality of printheads mounted on a scanning carriage is described, which includes printing an alignment pattern on a print medium, the alignment pattern comprising a set of target marks printed using a subset of the nozzles on each of the plurality of printheads, and a set of monochrome marks printed using a subset of the nozzles from only one of the printheads, each of the monochrome marks positioned in a nominal position with respect to a corresponding plurality of the target marks, and optically scanning the alignment pattern to determine measurements of relative positions between the monochrome marks and corresponding target marks. In one embodiment, the target marks include an elongated first mark disposed in a scan direction of scanning movement of the scanning carriage, and an elongated second mark disposed in a media moving direction transverse to the scan direction. In another embodiment, the target marks are patches of similar size to the monochrome marks.