Eve Arnold's talent for bringing out hidden depths in her wide- ranging subjects has made her one of the world's most sought-after photographers. She tells Mark Irvine why there is still work to be done

When Eve Arnold smiles, her face creases like crushed rice paper and you become the instant grandson. There are tired shadows under the twinkling eyes. "A mildly hysterical woman buzzed my bell at 4am this morning, saying she'd forgotten her keys. So I let her in. Later it transpired that she was a vagrant, and had got into the apartment downstairs. The police were called and my neighbour has been screaming at me saying the hall will need fumigating to get rid of the fleas." Her voice rises in protest: "But if you've got any kind of humanity, then what are you supposed to do at that time of the morning?"

During a career spanning 45 years, Eve Arnold, 83, has become an expert witness to the broad spectrum of humanity. Her camera has captured every expression, from senatorial triumph and poverty-stricken despair to malnourished resignation and calculated film-star ecstasy.

This diverse range, visible in a current exhibition of Arnold's work, "In Retrospect", might perhaps be used to support the cliche that the camera never lies. Arnold sits bolt upright in a sudden release of energy and laughs: "The camera romanticises because it isolates the image. You choose carefully what you're going to say about a subject. This can create a distortion of what the subject means, but this is not the same thing as a lie." Does this mean that the camera can never be truly objective?

"Look, the camera seeks actuality more than truth. John Houston said the only word he could think of to describe Marilyn Monroe was `truth' - which I've never quite understood. Maybe what he was saying was that she could never hide anything. Marilyn knew more intuitively about photography than anyone, because what you saw was what she wanted to show you. But it was always more than anyone else would show you."

Arnold developed a close relationship with the star over the 10 years they worked together, and I wonder if, for all her evident enthusiasm in talking about it, she secretly resents being known as the Woman Who Shot Marilyn?

"My God, am I only going to be known for that? I've done other things, you know. Serious political stuff in Afghanistan, Russia and China."

Looking at her diminutive frame, you wonder how she managed to deal with the hurly-burly of warfare, the scrum of the press pack, the ever-present danger in some of the world's most notorious hotspots.

"She's formidable and knows how to push in order to get to the front of the queue," says Michael Hoppen, owner of the eponymous photographic gallery. "There's an honest dialect in her work. She communicates with the people she photographs - she doesn't hide behind her camera."

Susan Sontag wrote in her book On Photography that the photograph is an act of intervention, that it gets in the way of the subject. She gives a qualified agreement, insisting on the indirectness of her art. "Eyes seek eyes, but I'm looking down the lens, so I get the person to do something else."

For Arnold, photography is an instinctive business. There are, she says, so many things that can go wrong when taking a picture, but high on her list is the fear that the subject feels they're not looking their best. Men, she explains, are especially aware of this. "Even though the English are very shy about being photographed, they then really get into it. And strangely, they'll also talk at the same time, which I don't find anywhere else. I get the most incredibly personal details - even stories about lovers - I'm often just boggle-eyed at what I hear."

Most of the time, however, her subjects are slightly apprehensive of the authority of the still camera - the "for all time" factor. "But then listen to the words used to describe photography: grab, shoot, capture - they're all provocative, aggressive words. It is an invasion, and I'm always irritated with myself and slightly ashamed that I'm taking something. Primitive peoples feel this, you know," Arnold adds, biting her lip - "the Hopi Indians ripped my camera apart once because they said I had taken their soul."

Other subjects, like Joan Crawford, have turned the tables and revealed themselves so brazenly to Arnold's camera that the photographer has felt obliged to return the pictures.

Again and again in the catalogue accompanying the exhibition she tells of her encounters with figures leading impossibly media-rich lives, all of whom engage in degrees of power-play with her. Public people, ordinary people, reacting to or being caught by the eye of one the world's most watchful faces. It's like reading an alternate, secret history of the mid-to-late 20th century.

It's impossible, Arnold claims, for the photographer not to want to impose a viewpoint on the subject. "There have been times when I've set out to show people to be what they are - like Joe McCarthy, who held America for ransom for many years and ruined a lot of lives."

So the photographer is a political creature?

"We're all political agents. How do you divorce yourself from the system you live in? You have feelings - it's not just wind whistling between the ears."

Arnold works close up to her subject and scorns the use of the long lens so essential to many of today's press photographers. "What they do has nothing to do with intuition," she says with calm contempt. "I don't regard that as photography because all you're doing is making a record shot. It is very different when you're working with someone."

And what of her family? She speaks with evident pride of her grandchildren. Has she photographed them all? "Yes, but only until they start to pose. Then I stop." But what about her husband (the end of Arnold's marriage is mentioned only briefly in the book)? She grimaces. "No. I find it difficult to photograph the people I love," she says with slight tetchiness. It is perhaps hard for us in the Nineties to know how people in the Fifties and Sixties would have regarded a mother dashing around the world taking pictures. You pay a terrible price. But you also deserve a life, and I thought I had something to say."

She still does. As part of Magnum's 50th birthday celebrations over the next two years - Arnold has been with the prestigious photo agency through thick and thin - she's been given an open-ended assignment to go anywhere she wants. She has decided to return to a small fishing community in Cuba, where in 1954 she photographed Juana, an eight-year-old girl.

"I remember seeing her from the small plane as she shinned up a coconut tree. It seemed idyllic - blue skies, blue Caribbean, waving palm trees. But they lived in hovels. At the end of my three-week stay, her parents begged me to take her with me. All her future had in store was poverty or prostitution in Havana. I had to refuse. It's troubled me for years."

In one of Arnold's pictures, Juana is tracing her dream house in the sand in front of the derelict shed in which her family lived. "She's now 50, has had eight children, and lives in a cinderblock house Castro built. She wrote to tell me that she now lives like a king," recounts Arnold, smiling broadly. "So I reckoned I'd go and find her."

`In Retrospect' continues at The Scottish National Portrait Gallery, Edinburgh (0131-556 8921) to 16 Nov; then runs at the Gallery of Photography, Dublin (00 3531 6714654) from 15 Dec