Only Paris can compete with New York as a center of photography. In the middle years of the 20th century, many photographers came to Paris from abroad — Man Ray, Brassaï, Lee Miller, Robert Capa — but a remarkable cohort of native Frenchmen added to a tradition that went back to Louis Daguerre’s discovery of photography in 1839. One of the loci of photographic activity in Paris was Magnum, the cooperative agency of photojournalists founded after World War II, which Marc Riboud (born 1923 in Lyons) joined in 1952. Over the next quarter of a century, Mr. Riboud went from Magnum to China, India, Vietnam, Africa, America, Cuba, and other distant lands, where he took some of the best-known photographic images of that era. Several of those, as well as pictures taken as recently as last year, are now on display at the Howard Greenberg Gallery in “Marc Riboud: Home on the Road.”

Mr. Riboud’s first published photograph (in Life magazine) was taken in Paris in 1953: It is the picture of an insouciant painter at work high above the capital city on one of the girders of the Eiffel Tower. It seems perfect. The girder goes up the left side of the frame, and two crossbraces form a triangle with the painter in the middle. He stands on one cross-brace and holds on to the other while he paints the girder in a posture that seems Chaplinesque. Or, in his porkpie hat, he might look more like Buster Keaton. At any rate, it is the combination of the vertiginous height and his nonchalant indifference to it that gives the picture a humorous cast and fixes it in memory. But the gallery has four other frames of the same subject on display, and the differences are instructive.

In one image, there is a little space between the solid upright girder and the left side of the picture, as a result of which the triangle is not such a perfect frame, and the painter is turned to the right, headed into the angle of the crossbraces; his brush is not visible so it is not clear what he is doing there. In another, the girder is gone and the painter stands on a section of cross-braces that seems to be floating in the air: This painter does not have the porkpie hat, so the element of humor is gone. In another, the painter is crawling cautiously along a cross-brace, and in another it is not clear what he is doing except holding on. These pictures illustrate one of Henri Cartier-Bresson’s maxims: that looking at a series of images on a good photographer’s contact sheet is like watching a carpenter set up a nail; he will tap, tap, tap at it, then finally smash it home.

Cartier-Bresson was Mr. Riboud’s mentor at Magnum, and must have appreciated how the young photographer worked around his subject on the Eiffel Tower until he got the image that was just right. Several other pictures at Greenberg have that feeling of just-rightness. “Watergate” (1974) is more than a news photo from the hearings in the aftermath of the scandal; it is a character study of President Nixon’s young White House counsel, John Dean, trying to grasp that he has blown his brilliant career, and of his sleek but appealing wife Maureen, no less rattled by the couple’s fall in status. In “Darjeeling” (1956), the trees beautifully frame strollers with umbrellas as they disappear down a hill into the mist. “Huang Shan: Mountain of Chinese Painters” (1985) is a chromogenic print bled of color so it resembles those ink drawings of improbable landscapes shrouded in clouds that seem more the products of the painter’s fancy than of any reality.

Mr. Riboud photographed many important people in his travels, especially during his extended stays in Asia. “Mao Tse Tung” (1957) shows Chairman Mao, the “Great Helmsman,” eight years after the creation of the People’s Republic of China, and a year before he launched the disastrous Great Leap Forward. The 64-year-old Mao looks like an all right guy; he stands at a ceremony, with dramatic light coming from the right, the side away from the camera, wearing his simple tunic, with one hand held in the other, and an incipient smile on his face. There is a prominent mole on his chin. A man, standing behind him and to the left, looks at him with practiced reserve. It is a bit unnerving to see Mao, a man responsible for tens of millions of deaths, shown as just another politician.

The same problem confronts us in “Ho Chi Minh, president of North Vietnam, a few months before his death, with Prime Minister Pham Van Dong, Hanoi” (1968). Uncle Ho and his prime minister sit on tasteful bentwood chairs at a matching table with a bowl of flowers on it. They appear to be in a formal garden, surrounded by crushed stone and a ring of trees. The dictator wears his tunic open at the neck and seems a bit frail. But he appears to be alert as he talks to Pham, who leans forward and makes a show of paying attention. Ho, in fact, looks avuncular and also like the Paris-educated scholar he was. But seeing him I think of the mass graves in Hue, the re-education camps, the thousands of boat people in their frail crafts, and the genocide awaiting the people of Cambodia, and there is a dissonance I cannot resolve between what I see and what I am looking for.

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