EVEN WHEN they lie, photographic portraits tell a particular and penetrating sort of truth. If it seldom captures soul the way the painter's brush can, the camera's unblinking eye almost unfailingly shows up sham.

Or perhaps, by presenting the appearance of being "real" rather than an artist's interpretation, a portrait photograph allows us to read whatever character we like into the sitter. Either way, photo portraits have a unique power, never better exhibited than in the National Portrait Gallery's current show of 150 photographs from the vast collections of its British counterpart, the National Portrait Gallery in London.

Embracing the entire 150-year history of the medium, the portraits are individually fascinating and collectively serve as a review of the rise and retreat of the empire. The contrast between, say, prime ministers Benjamin Disraeli and Margaret Thatcher gives substance to Kipling's complaint that "There's something gone small with the lot."

It would hardly be possible to pack more poignance into a single scene than "Queen Victoria and Her Descendants" (1899). At center sits the crusty old Queen, with two years more as mistress of history's greatest empire. Above her looms her fat and foppish son, soon to become the dissolute King Edward VII. To his right stands his son, the dutiful, earnest and ineffectual future King George V. Peering at the camera with precocious cunning is his son, who as King Edward VIII would renounce the throne for his American lover, would be banished for playing at treason during World War II, and would live out his life as a rent-a-duke, the world's most fatuous and indefatigable party animal.

But if there's melancholy in such sepia scenes, there's sparkle in David Bailey's 1988 portrait of Lady Diana, which cuts through all the nonsense of Fleet Street's frantic antics and shows her to be a woman of wit, poise and inner strength, not to mention zaftig. The late Norman Parkinson's 1963 portrait of the Beatles offers considerable insight into these bouncy boys who were about to make musical history.

The collection is particularly strong in that classic British type, the ineffably odious young man of irresistible talent: W. H. Auden, Aubrey Beardsley, Cecil Beaton, Richard Burton, Aldous Huxley, Christopher Isherwood, D. H. Lawrence, George Bernard Shaw, Oscar Fingal O'Flahertie Wills Wilde.

One of the most powerful and revealing portraits is by Simon Lewis of Sir Clive Marles Sinclair, a shy, geeky fellow almost no one will recognize, although the fruits of his genius surround us and profoundly affect modern life. His inventions include the pocket calculator, the cheap yet super-accurate digital watch, the miniature television and the low-cost home computer. Please go away, Sir Clive's image pleads. I can't deal with this.

A portrait of Amy Johnson (1903-41) evokes the very essence both of British pluck and the islanders' art of muddling through. Johnson, daughter of a Hull herring importer, was obsessed by airplanes. In May 1930 she made her first flight from London to Hull; her second flight was to Australia, solo, in 19 days in a tiny Gypsy Moth biplane. She was killed in World War II while flying war materiel transport.

The exhibit's concise, complete captions come from the text of the handsome catalogue ($35) by Malcom Rogers, who can encapsulate a character and characterize an era in a paragraph. It's a jolly good show.

CAMERA PORTRAITS: Photographs From the National Portrait Gallery, London -- Through Feb. 18 at the National Portrait Gallery, Eighth and F streets NW. Open 10 to 5:30 daily. Metro: Gallery Place.