"Man Drawing a Lute", woodcut by Albrecht Dürer (1525) Location: Metropolitan Museum of Art, New York wikipedia 
"Man Drawing a Lute", woodcut by Albrecht Dürer (1525)—wikimedia
"Man Drawing a Lute", woodcut by Albrecht Dürer (1525)—wikimedia
In the image, the point of view is the loop screwed into the wall. The string is a ray of light reflecting from the lute to that point of view. The apprentice holds the point on the lute that is to be drawn, and also holds the drawing itself away from the frame. The artist is marking the location of the point of intersection between the ray of light and the picture plane. On the drawing in the hand of the apprentice, we can see the foreshortened image taking shape, with geometric precision, dot by dot.