Leibovitz is 44, unassailably established, yet maybe pondering a change of gear, or wondering how things might have gone in other circumstances. Certainly in a decade of chiefly black-and- white assignments for Rolling Stone, you see in embryo another kind of photographer altogether. The shot of the country singer Tammy Wynette taken in 1971 is richly satirical: Wynette smiling openly on a prim porch, with a prim pooch, the manicured lawn receding to the shiny car - a scene on to which the baby tucked under her arm pours puffy-faced scorn. In the context of this exhibition, it's an unusual picture because it seems to contain a dialogue. What Leibovitz has done over the last decade or so is to banish that possibility from her work.
When she started with Vanity Fair in 1981, she gave up taking people in the act of doing what they do or being what they are, and instead concentrated on producing hugely contrived fantasies on the way she saw them, or the way they saw themselves. These set-ups are saucy, sexy, occasionally funny (Arnold Schwarzenegger sucking a fat cigar, his thighs clamped round a veiny stallion; Whoopi Goldberg sunk in a bath of milk), but they are always flattering. Even the people in potentially unflattering poses are flattered for taking the risk. With Leibovitz, you can literally dive into the muck, like Lauren Hutton, or Roseanne and Tom Arnold, or Sting, and still come up smelling of roses. No wonder celebrities love her so much.
From black-and-white she went to colour and to pin-sharp focus and wantonly artificial lighting - California enriched by halogen. David Hockney is so vividly back-lit that he appears superimposed on the Los Angeles view. Jodie Foster, glowing in the side-lights, stands against the sunset like an iron-on transfer. These images are designed, no expense spared, to push out at you; this exhibition houses the most expensive set of publicity stills in the world.
There's no doubting how well these pictures do their job, no questioning their allure, their glossy attraction. (The one we know, perhaps, better than any - Demi Moore, naked and heavily pregnant - isn't on the walls at all: a peculiar omission.) But then we are predisposed towards staring at famous people, so the question of a Leibovitz portrait angling for your interest, cunningly drawing you in, doesn't apply. For the most part, her techniques are, in fact, less than subtle. Take her love of a bright, singing red - magazine-spread red. There it is in Jodie Foster's dress and Ella Fitzgerald's two- piece and David Hockney's trousers and Muhammad Ali's carpet and the drape on Jose Carreras's bed. The shot of Sammy Davis Jnr, set against the Nevada desert, is a virtual silhouette but for the red handkerchief fluffing from his top pocket. This picture is no different from countless postcard images which briefly entertain the eye by recording an alarming occurrence of brightness. Except that the postcard-makers are by and large working with anonymous sources, while Annie Leibovitz gets to play with Sammy Davis Jnr and we accordingly give the picture a little more of our time.
The deal for the celebrities is this: nothing will be revealed that you don't wish to reveal - except, inadvertently, your lack of taste or your rampant egotism (consider Daniel Day-Lewis, shot quarter-on and dressed up as if posing for Gainsborough). As such, there is no tension in the relationship between sitter and photographer - only a flat sense of complicity. The pictures arouse a mild curiosity in their subjects (why was he / she interested in looking like that?) but more frequently they return you to the photographer - to the miracle of her power and access. What distinguishes these pictures in the end is not virtuosity or inspiration. It is the plain fact that they are taken by Annie Leibovitz. This is a neatly self-fulfilling routine: it will doubtless run and run.
Talking of access, the Annie Leibovitz exhibition is brought to the National Portrait Gallery by American Express. In what must be the most bizarre concession structure ever organised at a national museum, reductions on the admission price are available to the unwaged, but carriers of an exclusive credit card get in for nothing. There's a message in there somewhere.