Keystoning

Keystoning (Perspective Distortion): A geometric distortion where a rectangular subject—like a book page, document, or framed painting—appears shaped like a trapezoid, with one side looking visibly wider than the opposite side. This happens under a copy stand if your camera sensor is not perfectly parallel to the media stage. Because one edge of the document is physically closer to the lens than the other, the parallel lines appear to converge, ruining the dimensional accuracy of your digital copy and requiring software correction to square it back up.