How to create a Magnum opus

The success of a great photojournalist depends on two interdependent qualities: a nose for a news story and an unswerving eye for composition - the perfect juxtaposition of key elements at what Henri Cartier-Bresson called "the decisive moment".

To perceive subject and composition coming harmoniously together, look no further than Room 1 of "Eve Arnold: in Retrospect", currently at the Barbican Art Gallery. Here are her celebrated studies of Marilyn Monroe, with whom Arnold achieved a level of intimacy matched by no other photographer. Witness Monroe, shot from above, entering a press conference, her blonde head thrusting eagerly into the sea of grey-suited men as if diving into a pool. It's a brilliant piece of observation, with Marilyn (her face significantly obscured) representing but one small element in the image but revealing, in a single movement, everything about her starry aspirations.

Arnold is, understandably, somewhat weary of her Marilyn tag. "People remember me for Monroe," she says, "but that accounts for about three per cent of what I've done." And if anyone's still in any doubt about what she's done, this engrossing retrospective will put them right. It's a giant survey of a multifarious career that has taken on not only the Hollywood star system but American and British politics, the Civil Rights movement, the plight of single women in the Western world, and the veiled world of Tunisian women.

What shines through Arnold's photo-stories is the time and research invested in them, a luxury sadly denied most photographers in the cut and thrust of journalism today. But then Arnold was graced with a formal training by the great New York art director Alexei Brodovitch and, later, invited by Cartier-Bresson, Robert Capa and George Rodger to join their embryonic photo agency Magnum, now considered the pinnacle of photojournalism. This was the heyday of magazine photography and Arnold one of its finest pioneers.

It was Brodovitch who initiated her first assignment. Armed with a hand- held $40 Rolleicord, she covered a Harlem fashion show and the results, vibrant black-and-white studies of black models, made it into Britain's Picture Post later that year.

By the time Arnold settled in England in the 1960s and began contributing witty colour features on politics, religion, royalty and academia - with a line in British eccentricity that could only come from a foreigner - she'd already cut her teeth on the most prized 1950s American photo-opportunities. Access to Hollywood stars in the twilight of their careers threw up gorgeous, smoky, black- and-white studies of a relaxed and deeply sexual Marlene Dietrich during a recording session, and semi-surreal close-ups of Joan Crawford revealing, for the benefit of her fans, the rigorous beauty rituals she inflicted upon herself to maintain her status as a Hollywood icon.

But Arnold had access also to the political giants of the 1950s, like President Eisenhower, Joseph McCarthy and Malcolm X. The latter allowed Arnold the kind of proximity unheard of for a photographer who happened to be both white and female. Her study of the great campaigner collecting funds for Black Muslims in Washington DC exudes all the excitement, vitality and passion you'd expect. But, by the same token, her images of migrant children in Montauk, Long Island - especially of a small girl, dwarfed by the doorway that frames her - reflect, heart-achingly, all the uncertainty of the displaced.

This is one of the great joys of Arnold's work: her style is ever changing, her camera ever sensitive to the mood and personality of the next subject. For this reason, she manages brilliantly to blur the boundaries of glamour and reality. The heavily made-up bar-girl, resting her head on the counter of a Havana brothel, could be a star in a 1940s film, while Marilyn Monroe, her dress hitched up to reveal her knickers, fixing her hair in an airport waiting-room, could be a call-girl.

Arnold has been criticised for offering a softer view of the world than that of her male counterparts at Magnum, but this seems to be wholly irrelevant. Where Arnold eschewed the battleground, she showed us the no less painful and often violent reality of the hospital delivery-room, where new-born babies are held upside down and shaken, to add to the shock of new life.

But perhaps Eve Arnold's single most laudable quality has been her steadfast integrity in the face of this most competitive of arenas. When a half- drunk Joan Crawford insisted on posing nude at the end of their session, Arnold destroyed the negatives the moment she saw them. The 50-year-old star, she considered, had revealed quite enough of the ageing process, with close-ups of baggy eyes tugged about with eyelash curlers. She was, concluded Arnold, not deserving of such indignity.

n 'Eve Arnold: In Retrospect' continues at the Barbican Art Gallery, London EC2 (0171-638 4141) to 18 August