PORTLAND, Maine -- It's among the great stories in journalistic history -- photographic history, too. One day in April 1947 three of the world's leading photojournalists had lunch at a penthouse restaurant atop the Museum of Modern Art. Fed up with artistically shoddy and financially negligible treatment from the publications that ran their work, they decided to form a photo agency. For a name, they chose something unusual. It not only sounded impressively Latinate, it summoned up associations with both champagne and Smith & Wesson's most powerful handgun.
Thus was born Magnum Photos Inc. The founding three would soon become four, divvying up the planet like conquistadors. Robert Capa took America, Henri Cartier-Bresson Asia, David ''Chim" Seymour Europe, and George Rodger Africa.
Magnum would go on to become the world's premier photo agency. (Its website, www.magnumphotos.com, offers a wealth of images and information.) Magnum's roster significantly overlaps with any Who's Who of 20th-century photography: Eve Arnold, Bruce Davidson, Elliott Erwitt, Josef Koudelka, Susan Meiselas, Sebastiao Salgado, W. Eugene Smith. It remains in operation today, with offices in New York, London, Paris, and Tokyo, and more than a hundred members.
Although Magnum still exists, the world in which it was born does not. That was a black-and-white world, one not yet inundated with images. Photojournalism had more than just cachet. It had a special and compelling purpose. It enabled average people to witness great events, distant places, and grand personages. Now there's TV, the Web, and cameras on digital phones. Color is the norm. If anything, we give ourselves up more than ever to the sheer intoxication of seeing. Yet no longer do we believe in the power -- or even the necessity -- of showing. Who needs visual mediation in an age of instant access?
''In Our Time: The World as Seen by Magnum Photographers," which runs at the Portland Museum of Art through June 4, includes a picture that offers a metaphor for this situation. Peter Marlow took it in 1978 in an African refugee camp. A sea of hands and faces can be seen trying to capture the photographer's attention. Everything merges into a beehive-mass of motion and body parts, a democracy of visual clamor. Nothing stands out; everything blends together.
As technological change has transformed the media world, it's created a radically altered landscape for photojournalism. It's only fitting, then, that even though Magnum was, and is, dedicated to the moment, ''In Our Time" should take the form of a historical tribute. All 100 images are in black and white, with none more recent than 1988.
The photographs are also big, 16 inches by 20 inches, which emphasizes impact over artistry. This can make the show aesthetically fatiguing. But the larger size honors the photographers' original intent. These pictures were taken for immediate effect, the photographers having to assume their images would be shown small, surrounded by type and ads.
''In Our Time" includes the work of more than 40 photographers. Erwitt, that deadpan master of the incongruous, has the most, with seven. One of them shows Nikita Khrushchev's Kitchen Debate with Richard Nixon. The Russian premier's eyes are closed, as if he's thinking ''I just want to get out of here" (and who could blame him?). His lowered lids are what separates the picture from a standard news shot. Any photo editor on deadline would have immediately eliminated that picture -- closed eyes are seen only on the sleeping or dead. But for that very reason, the image leaps out. It's what makes this a Magnum photo rather than just another news photo.
Any self-respecting photo editor would also have rejected Burt Glinn's picture of Khrushchev at the Lincoln Memorial. It's shot from behind, so the caption would have had to do the work of identification. But ID'ing Khrushchev is irrelevant compared to the shock of seeing him appear to identify with the Great Emancipator. The way Khrushchev, bald head crisply in focus, stares at Lincoln, looming hazily above him in marbled splendor, you'd think he was Jimmy Stewart in ''Mr. Smith Goes to Washington." (The slump of his shoulders helps, too.) In the best photojournalism, the visual whole is vastly greater than the sum of the visual parts. The whole here is no less than the Cold War itself.
Glinn has three photos in the show, as does Eve Arnold, who turned 93 last month. Her pictures demonstrate a dizzying versatility: Joe McCarthy, Roy Cohn, and G. David Schine gloat in three-quarter profile; Malcolm X magnificently glowers behind a lectern; and Marilyn Monroe, wearing only a sheet, peeks over her shoulder and the arm of a helpful hairdresser.
Such versatility is a key aspect of photojournalism. Another is immediacy. Raymond Depardon's ''Christian Falangist fighter, Beirut, Lebanon, 1978" is news photography: on the fly, under fire, in the thick of things. It's news not as general event but specific instant. Then there's Ian Berry's 1985 photo of Archbishop Desmond Tutu: The indistinctness of his outflung hands makes visible -- palpable, even -- the urgency of his message.
A different kind of immediacy is on display in Dennis Stock's ''James Dean in Times Square, New York City, 1956." Watch Dean's movies today, and it's hard to understand what all the fuss was about. Look at Stock's photo, and you understand.
Mark Feeney can be reached at email@example.com.