"St Mark The Evangelist" by Andrea Mantegna (1431-1506)

In this trompe l'oeil painting, perspective is used to make the the figure appear to leaning towards us through an archway. A powerful illusion of depth is created by framing the head and shoulders with the other elements. Hold up your hands to crop these off, and the head/hand/shoulders suddenly looks like many a 'normal' portrait.