Innocent X did not want the portrait to make him look good. He wanted it to make him look right. Velázquez, who had arrived in Rome in 1649 for his second visit to Italy and stayed nearly two years, understood the distinction. The portrait he produced is one of the few full-dress papal images in which the sitter's expression is not an idealisation. The pope is seventy-six years old. He looks suspicious, tired, slightly amused, and entirely lucid. When Innocent saw the finished painting he is reported to have said: troppo vero, too true.

The technical work supporting the expression is the painting's quieter achievement. The painting is composed almost entirely of three reds, set against a fourth: the deep crimson of the curtain behind the pope, the brighter scarlet of his cape (mozzetta), the rose-red of his upholstered chair, and the cooler white of his rochet. Four colours, all variants of the same colour, modulated against each other with the kind of confidence almost no other portraitist of the period could risk. The result is a painting that vibrates without rhetoric. The viewer's eye cannot rest; the surfaces refuse to settle into a hierarchy.

The letter in the pope's right hand bears Velázquez's own signature, in tiny script, as if the painter were addressing the pope through the document the pope is holding. This is a remarkable conceit. The portraitist signs his work not in the conventional place (a corner, the back of the canvas) but inside the fiction of the portrait, with the sitter's hand involved in the signing. Velázquez and Innocent are co-authors of the document the viewer is being permitted to see.

The painting stayed in the Pamphilj family and hangs today at the Galleria Doria Pamphilj in Rome, in a small octagonal room dedicated to it alone. Francis Bacon painted six dozen studies after it between 1949 and 1971, the so-called screaming-pope series, transforming Velázquez's lucid sitter into a figure of contained scream. Bacon's project required the original to function as a stable reference point against which his distortions could register, which is one way of saying that the Velázquez has the kind of solidity that survives being argued with. The portrait has now spent three and a half centuries being looked at; it is no less arresting than it was on the day Innocent received it.