Glamour of the Gods: Hollywood Portraits, National Portrait Gallery, London

In the newly smart area of St Pancras there are currently two photographic exhibitions of black and white street scenes from the past: Walter Joseph's London of the 1940s and Vivian Maier's New York of the 1950s (reviewed here last week). Both enthusiastic amateurs focused on the humdrum details of everyday life.

At the National Portrait Gallery, we have the opposite. In the four rooms of the Porter Gallery are four decades of artifice: 70 artfully composed portraits of movie stars by studio professionals. All the big names of glamour are here, from Rita Hayworth to Elizabeth Taylor, draped on couches or mid-clinch with their on-screen partners. Many shots have not been shown in Britain before.

They form part of the archive of John Kobal, a collector who became interested in Hollywood portraits before anyone spotted their artistic importance. The complete collection is in the catalogue, well worth the £25.

These images were essentially film publicity shots, given away copyright-free to fans and magazines. They presented to millions an illusion of perfection, and catapulted actors to godlike status. This was before anyone had the idea of papping a celebrity falling out of a taxi, and then drawing red circles round her cellulite.

But then it's hard to imagine the beauties of the 1920s exposing their tit tape. The fashion then was for snub noses and boyish haircuts, as we see on Lillian Gish and Clara Bow. Or flapper pearls and severe bobs, as in a noirish silhouette of Louise Brooks, taken by Eugene Robert Richee.

Who? Nor me. But part of the show's point is to give credit to the unsung masters of Hollywood photography. Kobal tracked down many of them before he died in 1991. Some built up friendships with the stars they shot, such as Clarence Sinclair Bull with Greta Garbo, which perhaps is not surprising. Some of these pictures are so elaborate they must have taken hours. Or did they? It's not until room three that we discover these pictures were touched up later in the lab.

The most interesting image isn't on the wall. It's tucked in a cabinet and is of Charles Boyer and Irene Dunne, marked up for retouching. A backroom worker has criss-crossed the areas that need work – stubble and hairy hands for him, crow's feet and a flabby jawline for her. If you have been duped into believing in the fantasy of Hollywood, this snap brings you down to earth. It's like finding a page from Heat in a copy of Vogue.

And it makes you think somebody has slightly missed a trick curating this show. Because although nobody could ever tire of gazing at Grace Kelly or Garbo, it does not tell us anything new. In the same cabinet as the Boyer/ Dunne shot, there are pictures of the sets on which these snaps were taken. Bald men scurry around as divas wait in their poses, shattering the illusion that glamour is effortless. Seeing the wider picture certainly kills off the god delusion, but as the amateurs Walter Joseph and Vivian Maier show, truth can be just as interesting.

Until 23 October