A printer includes a carriage configured to move a printhead along a carriage scan path; a photosensor that is mounted on the carriage, the photosensor including a field of view; a light source; and a light guiding element having a first end that is aimed at a first predetermined position along a media advance path, and a second end that is aimed at a second predetermined position along the carriage scan path, wherein the carriage is movable to an edge-detection position such that the second end of the light guiding element is aimed at the field of view of the photosensor.