Sitting in the basement of his agency in Paris, Sebastiao Salgado is recalling the camera that changed his destiny. The memory is more than three decades old, and yet still vivid. There is a glint in his bright blue eyes, his Picasso-like bald head is leaning across the table, his bushy white eyebrows are raised and he is repeating his favourite adjective – “enormous”.

“My wife bought it when she was studying architecture in Paris,” he says in a French softened by his lilting Brazilian accent. “I had never taken a photo in my life but when I handled that camera and looked into it, I got enormous pleasure.”

He was on course for an academic career at the time, completing a PhD in France before moving to London as an economist with the International Coffee Organisation. Many of his assignments were in Africa and the camera – he had acquired his own Leica by now – always went with him. “The pleasure I got from it was enormous,” he says again. “So enormous, in fact, that I resigned from my job and became a photographer.”

But not just any photographer. According the French newspaper Le Monde, Salgado is the world’s most widely viewed exponent of his art. He has filled more magazine pages and had more exhibitions than anyone else alive today. His shots of mud-soaked Brazilian gold-miners, of glistening oil workers in Kuwait after the first Gulf War, of terror-stricken Rwanda refugees, are among the most recognisable of our times.

In short, Salgado, 64, is an institution. The sixth child of a Brazilian rancher, he spent his early years on the family farm before becoming involved in left-wing activism and having to flee his country’s military dictatorship in 1969. He moved to France and married Leila Wanick Salgado, whom friends say is the driving force behind his success and the organiser of his book and shows.

She gave up architecture after the birth in 1979 of their second son, Rodrigo, who had Down’s syndrome. In 1992, they created Amazonas Images together, “perhaps the smallest photo agency in the world because there is only one photographer,” he quips. Now, 15 years later, Salgado is returning to his roots.

His latest exhibition, at Gallery 32 in Green Street, London, stems from a journey back into the world of coffee. Sponsored by the Italian coffee maker Illy, he travelled to Brazil and to Guatemala, India and Ethiopia, to shoot workers along the trail that leads to the espresso consumed – the company claims – in 50,000 of the world’s best restaurants. The title of the exhibition is In Principio (In the Beginning) – a double reference, presumably, to the origin of Salgado’s career and of Illy’s coffee.

Among the images, a Brazilian is hidden behind beans rising in an oval from his sieve, an Ethiopian works seeds with long, fine fingers, Indians are glimpsed behind a sea of sacks which they are loading into a lorry. The traits that run through Salgado’s work are present: shafts of early evening light, heavy skies, an almost renaissance beauty picked out of poverty in black and white.

But what is he doing in a commercial partnership with a profit-making business? His two most famous books, Workers and Migrations, were a harrowing account of today’s world, with its famished refugees and downtrodden labourers. Many saw them as a powerful attack on a system. Has he now joined that system?

He adopts the caution of a world-weary, media-savvy operator. “I do a lot of work just to earn a living,” he replies. “In the UK I have even done a lot of advertising shots for people like Volvo and Saatchi & Saatchi.” He never intended to cast himself as the social conscience of the West, he says. “I have no claim to be a social photographer. People stuck that label on me, but I do a lot of commercial work like everyone else. I am not a political militant, I’m a photographer and that’s all.

“I am from a poor country and I have spent a lot of time working in poor countries. I don’t photograph them to make the rich feel guilty. I photograph them because it’s my life, it’s what I like doing.”

So when Andrea Illy asked him to spend 15 days a year with the firm’s coffee growers, he saw no reason to refuse. Illy, after all, is not Nestlé, he says. It treats producers well, cares about the environment and has been responsible for “an enormous, enormous, enormous improvement in the quality of coffee in Brazil”.

He says the company gave him complete freedom. “I am not there to show that Illy is good or bad. It’s pure photojournalism. The idea is to show how coffee is made.”

Salgado knows what he is talking about, not only because he was employed by the ICO but also because his father had a coffee shelling plant on the farm he bought in the Rio Doce valley in south east Brazil in 1948. But there is another story to tell about that farm as well, a story of deforestation as Salgado’s family, like thousands of others, cut down trees to make room for sugar cane. To make amends, he and his wife bought back the same land in 1990 to found an environmental institute.

Since then, the Instituto Terra has planted almost a million trees, established a university that offers lessons in ecology to forest police, mayors and farmers, and contributed to what Salgado says is an unprecedented groundswell of concern.

“When we set out, people didn’t want to listen to us, they mocked us. But in October last year, we had a meeting with land owners and 487 came – which shows that people are now really with us.”

His green campaign is also shading his work. Despite his claim to be a photographer without political baggage, he set himself a mission to shoot the 46 per cent of the Earth which he says is still unspoilt. The project, called Genesis, has taken him to the Galapagos islands, to Patagonia and most recently, to the Okavango Delta in Botswana. “I want to show those parts of the world which are very pure and show that we have to preserve them,” he says. He has been working on Genesis for three-and-a-half years and has another four-and-a-half to go, by which time he will be 69.

The boyish effusiveness returns. “Photographers are lucky because they live to old age,” he says. “And I hope to carry on taking photographs right until the end. The pleasure I got from my first photos is still there. It’s an enormous pleasure.”

In Principio: A Photographic Journey through the Land of Coffee is at Gallery 32, London W1 (020-7399 9282, www.brazil.org.uk/gallery32), September 11 to October 6

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