Hundreds of people are swarming up ladders, scaling the cliff-like sides of a gargantuan, man-made pit. Is it a picture of hell? Some kind of spirit photograph showing life in the Aztec empire? In fact, Sebastião Salgado’s photograph captures gold-grubbers pouring up the side of an opencast mine at Serra Pelada in Brazil. One of a jaw-dropping series he took of the crazed gold rush that created this great hole in the Earth in the 1980s, the shot is bizarrely timeless and disorienting. Few photographs have such power – to make you question your assumptions about the world, to show you something unbelievable yet utterly real.

Salgado is a photojournalist who seeks out the most moving, unsettling, perspective-shifting images of life on Earth. From his mind-swarming images of the Serra Pelada gold mine to his most recent epic labour Genesis, which documents the last pockets of undamaged nature and unmodernised peoples on Earth, Salgado shows secrets from remote places: things you thought were lost, crimes you never imagined. There could scarcely be a better choice for a lifetime achievement award from Photo London, an art fair opening at Somerset House this week. In addition to an exhibition of the Genesis prints, it will feature works by, among others, Stephen Shore, Rut Blees Luxemburg, Vik Muniz and Ori Gersht.

Salgado is not just a great photographer. He may well be the last great photographer – at least in the classic, humane tradition, working in black and white, telling profound truths. You can leaf through any of Salgado’s books and every few pages be pulled up by a shot that seems like one of the best photographs ever taken. In Genesis, the eye of a whale peeps out of the sea, looking back at the photographer. A group of African herders move among long-horned cattle in an ethereal dust cloud. A baboon balances on a sand dune. These pictures rank with the masterpieces of Cartier-Bresson. But Salgado is not sure if his kind of serious photography can survive the digital age. Photography, he says, is now turning into something else: “Your father and mother, when you were a child, they took precious photographs of you. They went to the shop on the corner to get them developed. That is a memory. That is photography.”

The opencast mine at Serra Pelada, Brazil, 1986. Photograph: Sebastião Salgado

Digital photographs, he says, cannot have that treasured sense of embodied memory because a photograph “is not something material today – it’s inside a computer. You lose your phone, you’ve lost your photographs.” Worse, we no longer see photographs as documents but as things that can be manipulated with Photoshop, glamorised on Instagram. “Before, we took a picture. It was reality.”

Even before the birth of the digital camera, though, critics were questioning photography, suggesting it actually creates a rhetoric of the real. But Salgado proves it truly can reveal new dimensions of the physical, human world, the richness of the peculiar, lovely, terrifying, yet somehow universal things he sees through his viewfinder. “When I first took a camera, I had never looked through a viewfinder in all my life.” It was, he says, an electrifying moment.

Salgado was born on a farm in the state of Minas Gerais in southeast Brazil. He and his wife Lélia Wanick studied in Paris, where their studio is still based. It is very much a collaborative enterprise: Lélia designs the beautifully structured books through which Salgado tells what he calls his “stories”. More recently, they have worked together to replant a section of Brazilian forest on the farm where he grew up. Yet he never studied photography or art – he has a PhD in ecomomics. It was while travelling the world for the International Coffee Organisation in the 70s that he had that epiphany when he looked through a viewfinder and fell in love with photography.

Antarctica, 2005. Photograph: Sebastião Salgado

“The human animal is a political animal,” he says, and his humanitarianism is unmistakable in his pictures of crises and endangered ways of life. But he insists he is not motivated by “activism”. Instead, what he stresses is the sheer fun of what he does. Photography has taken him all over the world, has been his passport to see astonishing things, people, places. He remembers standing in the Kuwait desert photographing oil wells on fire after the first Gulf war. It was an incredible spectacle. “All these wells burning - all the adventure to be there.”

Looking at Salgado’s most memorable pictures – not least his hellish scenes of the blazing oil wells – you are punched in the stomach by reality. Special effects can create all kinds of cheap magic. Yet Salgado shows raw, real events that outdo digital fantasy. That shocking, sensational, or utterly poetic moment of truth is what Cartier-Bresson called “the decisive moment”. How does he catch them? To get great shots, he says, you “need a lot of time”. You have to put yourself where things are going to happen, and get into the flow of events, the mood of people. “You know where you’ll go, but you don’t know what you will bring back.” When the subject does suddenly appear, you have to be ready – and fast. “Photography is one 250th of a second,” says Salgado. And so much can go wrong. “There are a lot of variables. There must be light. There must be power. There must be personality if it’s a portrait.”

Sebastião Salgado. Photograph: Nicole Toutounji/Unicef

The picture is not being taken by a passive camera, though. It’s by the person holding the camera. And this may be what makes Salgado so special: the economics PhD, the political idealism, the connection with nature and the past that comes from his forest childhood – he somehow gets all that into the picture, somehow puts his soul into the image. “In this moment, you bring your history and your ideas to what is in front of you. That is a photograph.”

Salgado, in that instant when “photography freezes things”, transmits his own vision of a vast, suffering, interlinked planet. The grandeur of his crowd scenes is heartbreaking. People fade into the distance among the tents of a refugee camp. They process exhausted along a jungle road. They lie dead, uncountable, in his most horrific photographs of genocide in Rwanda. Always, there is a sense of depth and scale, of the hugeness of human history and the story of the Earth itself. He gets the superb depth of field that makes his pictures so haunting with techniques he has learned over a lifetime:

“I work with very fast film. I always close my diaphragm to give a huge depth of field. Volumes for me are very important.” The result, the expansive yet immensely detailed scope of his pictures, is what makes them so moving. The migrants are moving through a forest, too tired to think. Some have collapsed by the pathway. Behind them, so many more are coming. Fear fills the air. “Reality,” says Salgado, “is full of depth of field.”

• Photo London is at Somerset House, London, 21-24 May.