How Pop changed the portrait

Nowadays Pop Art is the new Impressionism - the lowest common denominator of modern art and a surefire way for galleries to up their visitor count. Despite this, Pop Art Portraits is a well-chosen and spirited show that occasionally falls victim to its own enthusiasm.

There are many interesting loans from America, including the work of lesser-known figures such as Mel Ramos, who sleekly sprayed pin-up girls lounging on candy and cuddling fizzy drinks.

There is also an appropriately large representation of British artists, not only the usual suspects - Peter Blake, Allen Jones, Paolozzi and Richard Hamilton - but also one of Pop's rare female artists Pauline Boty, now virtually the object of a veneration cult by contemporary women artists, and Gerald Laing, represented here by a striking painting of an astronaut, in which a newsprint photo of the head of a spaceman surmounts triangular canvases of cartoon flames.

Portraiture was at the heart of Pop in a way that it hadn't been in art for decades before. Instead of obsessing about paint texture, brushstroke gesture or basic bodily functions, Pop artists obsessed about the visual totems of their society. They wanted to capture in their works the hypnotic spell of instantly recognisable images from the media and adver tising industries - an aura which was easier to represent visually than explain verbally. This was a matter of both style and subject.

In the first instance, Pop Art portraits were old-fashioned portraits painted in the new pop styles. Pop used the commercial technologies of image production, thus Andy Warhol's self-portrait in this exhibition is a mechanical reproduction of a mechanical reproduction - a photo-booth passport photo, enlarged and screen-printed. Pop Art was about painting the real world in a contemporary way, so some artists took the techniques of contemporary abstraction and applied them to their portraits, as in Hockney's early work Portrait Surrounded By Artistic Devices (1965).

Others, such as Rauschenberg and Johns, invented new kinds of "metaphorical" portraits, paintings in which the sitter is represented by abstract paint, carboard boxes, newsprint and other bits of collage.

But there was also a second metalevel of Pop Art portrait, exemplified in the show's spectacular room of Marilyn Monroe pictures by some of the greatest names of Pop. These are paintings of photos of celebrities, a kind of portraiture about portraiture.

Claes Oldenburg evokes the emptiness of Monroe's media image by portraying the Hollywood actress as a series of hangers in a wardrobe, from which hang the skeletons of clothes made out of string. Richard Hamilton tinkers with photos of the star on a beach as if he was her PR agent selecting the right still for a magazine. On the far wall hangs Warhol's classic 10-screenprint set of Marilyn's head. Its ambiguous power derives from the aesthetic contradiction between the intensity of the colours and carelessness of the images' execution.

Pop Art Portraits is an exhibition that is far too small and full of holes - hello, where are the European Pop Artists? It also takes a rather generous definition of portraiture, and is on dodgy ground including a classic Lichtenstein cartoon and a Tom Wesselmann nude. Still, it argues with panache that portrait-painting was fundamentally redefined by Pop Artists.

From Holbein to Augustus John portraitists painted power and personality; after Pop they painted the representation of power and personality.