Like quarterbacks and chess players and investigative reporters, most so-called "straight" photographers do their strongest work when young. While the greatest of great painters, Titian, say, or Goya, develop old-age styles and instead of growing stale just keep on getting better, most photographers decay.

One such is Paul Strand (1890-1976), the pioneering modernist whose centennial retrospective is on view at the National Gallery of Art.

Had Strand died in his twenties, he still would be regarded as a master. But he kept on making pictures, and lived to 85.

He never lost his seriousness, his heartfelt leftist politics, or his wonderful control of the subtlest shades of gray. What faded was his power. And his early gift for prophecy. The first rooms of his show contain some of the most prescient photos ever taken, pictures that predict Walker Evans, Edward Weston, even Robert Frank. But then the show diminishes. The pictures in its last rooms matter less and less.

The National Gallery, for many years, paid only slight attention to the medium of photography. It is still extremely choosy. So far only five photographers have been allowed to enter its permanent collection. The first was Alfred Stieglitz (one of Strand's chief mentors); then came Ansel Adams (that meticulous adorer of the Western landscape), Evans, Frank and Strand. As much as any of the others, Strand can be regarded as a sort of pivot. He altered the development of photography-as-art.

His finest early pictures -- say, "Wall Street" (1915), "Blind Woman" (1916) or "People, Streets of New York, 83rd and West End Avenue" (1916) -- feel impervious to time. They might have been shot yesterday. And yet when they were taken, ladies still wore ostrich plumes and horses were still common on Manhattan's streets. It's their modernity that startles. They have an openness, a daring, that most of us assign less to street photography than to abstract art.

That monstrous round-eared shadow in "People, Streets of New York" is a kind of field painting. Those black rectangles in "Wall Street" are as ominously suggestive as Mark Rothko's last abstractions, or the black squares of Malevich, or the time-controlling monolith of Stanley Kubrick's "2001." Strand's best pictures moralize. That blind newspaper seller is not an entertainment, nor are those small folk crushed by Wall Street. Something in these photographs is meant to prod the conscience and to pierce the heart.

When he wasn't snapping stills, Strand made stiff and fervent films about wrongs done to workers. He went to Moscow in the '30s, and so detested Joe McCarthy that he moved, for good, to Europe in 1951. As late as 1965, Strand publicly refused a White House invitation in protest of the Vietnam War. The frivolous, the pampered, are excluded from his art. Trembling throughout his show is an adamant allegiance to what he liked to call "the plain people of the world."

He was not really one of them. Compared with other New York Jewish families, his was prosperous and privileged. At the age of 21, he spent six weeks touring Europe. Strand was raised in comfort, not in some Lower East Side tenement, but in an 83rd Street brownstone near Riverside Drive. He had a private education. At 14 he was sent, at considerable expense, to the Ethical Culture School on Central Park West. Strand's art is deeply ethical -- and slightly goody-goody. His workers are too noble, and his peasants are too dignified, to be quite believed.

Strand might never have become a political progressive -- or for that matter a photographer -- had not one of his teachers at the Ethical Culture been Lewis W. Hine. Hine was a ferocious fighter for the poor, and a master propagandist. His camera was his weapon. His fine and searing photographs of ill-fed, ill-clad kids in factories and mines did much to prompt the passage of child labor laws.

Strand had two great teachers, and both, in different ways, were moralist crusaders. One was Hine the devoted documentarian. The other (Hine introduced them) was Alfred Stieglitz, the hugely influential champion of photography and progressive modern art. He took quickly to young Strand, and greatly changed his art.

Most art photographers in those days still were making photographs that pretended to be paintings. Out-of-focus trees and out-of-focus ponds were then so much in fashion that the most advanced pictorialists sometimes kicked their tripods to increase the blurring. The oldest of Strand's pictures here -- "Venice" (1911), with its Whistleresque reflections, and "Cambridge, England" (1911), with its hay and grazing sheep on the meadow -- partake of that artiness. But Strand soon turned from mushiness. By 1917 he was writing manifestoes for the sort of crisp photography we now describe as "straight."

"The full potential power of every medium is dependent upon the purity of its use," he wrote, "and all attempts at mixture ... hand work and manipulation is merely the expression of an impotent desire to paint."

Strand fought to tell the truth, or what he thought the truth. He even took the trouble to put a fake lens on the side of his big camera so that he could take street photographs without his sitters knowing they were being snapped. Yet he was seeking more than candor. His photographs were intended to be seen as works of art.

Hine's people are just people. Strand's rather feel like statues. His "Blind Woman" is as monumental as a sculpture by Rodin. Strand also understood Picasso. His "Wire Wheel" (1917) and his "Chair" (1916), with their splinterings of space and unexpected angles, are nearly cubist pictures. And early on he accepted precisionism's cleanliness. His 1923 studies of the shining gears and wheels of his Akeley motion picture camera are pure prayers to machinery. The early studies that he made of lichen-spotted rocks are almost totally abstract.

Young Strand, though a modernist, was never a surrealist or, God forbid, a dadaist. He is never goofy. A classical intent, a deeply felt belief in order and correctness, is felt throughout his art.

His prints are free of flaws. While lots of other artists took to little lightweight Leicas, Strand retained his loyalty to the 8-by-10-inch Deardorff and the 5-by-7 Graphlex, to the black hood and the tripod. He preferred to print on paper coated with platinum, not silver. And he resisted the enlarger, instead relying upon contact prints in which every tiny shift of tone is rightly gauged, and clear.

The gallery's exhibit is subtitled "An American Vision." During World War I, with Europe drenched in horror, with soldiers by the millions dying in the trenches, Strand, like many artists here, fought to free his art from Europe, from its suaveness and corruption. His new work would instead be American to the core.

It would sing the American worker, the American land and language. Strand, from the beginning, was a member in good standing of what Sarah Greenough, the gallery's curator of photography who organized this show, calls "a loose coalition of cultural nationalists." Some of them were poets -- who scorned the antique thees and thous of Tennyson and Browning for the good plain speech of common folk. The painters of their company would paint our wave-washed shores (like John Marin), or our factories (like Charles Sheeler), or the clean soul of our deserts (like Georgia O'Keeffe).

In July, 1920, Strand sent a kind of fan letter to writer Sherwood Anderson. "... I have come to know 'Winesburg, Ohio' {Anderson's book of connected short stories}," wrote the young photographer, "and it has given me a still greater sense of a spirit, actively strong and sensitive, penetrating the crassness and brutality of the American scene, to a new beauty."

Strand's famous "The White Fence" (1916) is a hymn to that "new beauty." Its marching hand-sawn pilings, like patriotic soldiers, fight to bring the timeless grandeur of the Parthenon to an Upstate New York farm.

Much of Strand's best work is dependent on his sense of place. When he photographs Manhattan, or the shadows of his porch, we utterly believe him. It is when he starts to travel -- to the Gaspe Peninsula, and later to such far off lands as Ghana and Romania and the Outer Hebrides of Scotland -- that we begin to have our doubts.

His Gaspe clapboard houses are a bit too reminiscent of Andrew Wyeth's barns. The same goes for his Western ghost towns. Enough weathered wood already! Gradually, but steadily, Strand's modernism fades.

Great painters reinvent the world. Straight photographers, however, can't escape the given. Strand, as he grows older, begins to repeat himself. The plain white fence he photographs in Canada in 1929 adds nothing to the one he shot 13 years before.

And his portraits lose their punch. Strand is so intent on showing us (again) the dignity of common folk that all his sturdy peasants, his fishermen and farmers, begin to look like figures from the National Geographic. Strand's sombreroed Mexicans uncomfortably resemble those spouters of folk poetry who wear the white pajamas in Elia Kazan's "Viva Zapata." They all seem to be saying stuff like "Who can tame the river, who can catch the wind?"

Occasionally after World War II Strand manages to come up with a portrait that has force, that stabs. One such is his "Young Boy, Gondeville, Charente, France" of 1951, but late pictures of such power are few and far between. Strand lived in France for the last 25 years of his life. But he never learned the language. One knows that from his photographs. Though stringently composed, and printed with great care, they feel taken from outside.

Young Strand belongs to history. But old Strand is an afterthought. Writing in the catalogue, Michael E. Hoffman of the Aperture Foundation says Strand "had a genius, perhaps unmatched by any other photographer of this century." But that is overpraise. Strand may be a master after 1930, but if so he must be among the least impelling masters of his time.

The Paul Strand retrospective is in a large sense owed to Southwestern Bell. The corporation has not only funded -- with a $500,000 grant -- the present exhibition, its catalogue and tour; it has also pledged, or given, 61 Strand prints to the gallery's collection.

Greenough's catalogue deserves high praise, both for her text and for the superb quality of its reproductions. They were printed, by Franklin Graphics, using four ink colors on a six-color press, and they could not be much improved.

More than 30 lenders have supplied this exhibition. It will travel to Chicago, St. Louis, Houston, the Whitney Museum of American Art in Manhattan, San Francisco and the Victoria and Albert, London, after closing -- in the East Building -- on Feb. 3.