Imagine it: an afternoon alone with 150 of the most famous and fascinating Brits ever to grace the planet -- writers and royals, painters and politicians, singers and scientists -- from Queen Victoria to Princess Di, Anthony Trollope to Samuel Beckett, Lillie Langtry to the Beatles.

Imagine having them all to yourself for as long as you like, with license to stare, lock eyes and read the last 150 years of British history in their faces.

Reading faces, of course, is what National Portrait Galleries are all about. And two such institutions -- the venerable prototype in London, and its younger clone in Washington -- have brought us "Camera Portraits: Photographs From the National Portrait Gallery, London, 1839-1989," just opened at the National Portrait Gallery at Eighth and F streets NW.

Unlike most shows organized to celebrate the 150th anniversary of photography, this one focuses on portrait subjects rather than photographic history. The result: an experience more akin to a star-studded cocktail party than an exhibition. For sheer enjoyment, no show in town can beat it.

Plunging in, each visitor is bound to be drawn to different faces in the crowd, some as familiar as Aubrey Beardsley and Oscar Wilde, others as unfamiliar and surprising as the grandfatherly Thomas Hardy, and the haunted, rather scary looking D.H. Lawrence. But everyone will be stopped, sooner or later, by the image of portly Queen Victoria, empress of India, seated in her garden at Frogmore House, Windsor, being photographed as she reads dispatches, with her devoted Indian clerk standing by her side. With her feet propped on a cushion, a blanket on her lap and a parasol close at hand, she is formidable, yet unforgettably alive and vulnerable.

Victoria, who enthusiastically presided over the first great age of commercial photography, is surrounded here by a treasure trove of rare daguerreotypes, albumen prints, cartes-de-visite and other early photographs of great Victorians, produced in the proliferating photographic studios of the time. One portrays her dear, unhandsome prime minister Benjamin Disraeli, another the poet laureate Alfred Tennyson, who, despite a strong, noble face, looks distinctly unhappy about having to sit for his portrait. Florence Nightingale is here, along with Mrs. Beeton of cookbook fame. The bald, darkly bearded Charles Darwin is a complete surprise, far too wistful to have turned the world of belief on its ear.

Several portraits set up resonances with others nearby: a self-parodying photograph of expatriate American painter James McNeill Whistler hangs within shooting distance of his nemesis, art critic John Ruskin, from whom he won a farthing and near ruin in a celebrated libel suit. And playwright George Bernard Shaw, usually pictured as a wily old man, here cuts a startlingly attractive and manly figure in a photograph taken at age 36, just after he'd taken his first job as a music critic, and was obviously thriving on socialism and a vegetarian diet. This electric, sexually charged portrayal gives the portrait of the beautiful actress Mrs. Patrick Campbell -- who later became Shaw's great friend and correspondent -- a new dimension of interest.

So fresh and vivid are these portraits, so intriguing the personalities (each described in informative labels of remarkable concision) that it takes some time to realize we've been paying no attention to the high achievement of the photographers themselves. Many of the early ones are unknown to most of us, especially in America -- including George Charles Beresford, who produced what is surely the loveliest photograph in the show: novelist Virginia Woolf, shown here as an ethereal beauty in 1902.

Other photographers are well known, though not necessarily as portraitists: among them David Octavius Hill, Roger Fenton (who photographed the Crimean War), "Through the Looking Glass" author Lewis Carroll (a k a Charles Lutwidge Dodgson), French photographer Nadar, and Julia Margaret Cameron, whose offbeat portrait of Anthony Trollope makes him look like a a glowering rabbi.

As the 20th century unfolds, and modernism and photojournalism make stylistic inroads, several Americans join British photographers like Bill Brandt and Cecil Beaton in documenting Britain's new age. Some truly distinguish themselves: A mere puff of smoke in Lee Miller's portrait of documentary filmmaker Humphrey Jennings draws us from across the room to make his acquaintance; the sheer irrepressibility of Cornell Capa's portrait of a fat man tickling his dog, likewise brings about an introduction to the founder of the Glyndebourne Opera, John Christie.

But it is British photographer David Buckland who closes this show, albeit with an image deeply influenced by contemporary American art and culture. A billboardlike, color-enhanced portrait, it features two publishing tycoons: Vanity Fair editor Tina Brown and her editor husband Harold Evans, who loom darkly and ominously over the New York skyline. Photographic portraits have come a long way since the start of this show, and so, indeed, have the photographers who make them.

It comes as a shock to learn that recognition of photography as a valid form of portraiture, equal to painting and sculpture, came so late. The British Portrait Gallery, though it had been given thousands of photographs since its founding in 1856, paid little attention until 1972, when it set up a separate curatorial department to tend and collect photographs. At our much younger National Portrait Gallery, founded in 1962, photographs weren't even permitted in the collection until a special act of Congress changed that rule in 1976.

Meanwhile, it appears that while the aesthetes brain-wrestled over whether photography was an art, photographers simply went about the business of making some of the most vivid and telling portraits of their time, as can be seen in this splendid show. Its catalogue, by the way, includes all the photographs on view and the complete text, and would be a welcome sight under any Christmas tree.

Mobil supported both the exhibition and the catalogue, and noontime and evening lectures are planned for January thanks to the British Council. Phone 202-357-2729 for details and reservations.

"Camera Portraits" continues through Feb. 18 at the National Portrait Gallery, which can be easily reached from the Gallery Place Metro stop. Hours are 10 to 5:30 daily; closed Christmas Day.