All great photographers start somewhere. Widely regarded as one of the world's most eminent image-makers, James Nachtwey began his career in the early 1970s. Fresh out of university, the American art history and political science graduate embarked on a journey that would see him document some of the most impactful global events of the past four decades.

The young photographer did not have any formal photographic training. All he had was a deep, unwavering conviction that photography was his calling. "I believed in it, and I believed I could do it," James told budding photojournalists during a talk at a Canon Student Development Programme seminar at the 2022 Visa pour l'Image International Festival of Journalism. "So, I began to teach myself. I rented a darkroom space, I borrowed a camera, I learnt how to develop a film and make prints.

"I would go to bookshops and stand in the aisles looking at books by great photographers, putting them back on the shelves because I didn't have enough money to buy them. I had great mentors who I'd never met, and I learnt from them by studying their work."

It took 10 years, he says, "before I felt the least bit qualified to become a war photographer, which was my ambition". James has since been awarded the Robert Capa Gold Medal by the Overseas Press Club of America five times, the World Press Photo of the Year award twice, and has been recognised with numerous other accolades for his exceptional coverage of the events that shape our world.

After cutting his teeth as a newspaper photographer, James went freelance in 1980, spurred on by images of the Vietnam War and the American civil rights movement. LIFE photographer Larry Burrows was a huge inspiration for James, as he found his images to be "tremendously moving and informative".

James' belief now, as it was early in his career, is that photojournalism is a powerful tool that can bring about positive change in the world. "I was driven by the idea that a war photograph could become an anti-war photograph," he explains. "An image of social injustice could become an indictment crying out for change. I believed people would care if photographers showed them something worth caring about. My motivation was to use photography to say something about, and to ask questions about, what's happening to people."

If creating public awareness about crucial social issues is and always has been his goal, he concedes that the press is the most effective way to do this. And yet, having autonomy is vital. "I choose the subjects I photograph, and the way I photograph is my own," says James, who has been a contract photographer with TIME Magazine since 1984. "It's not dictated or influenced by editors or by any third party. It's based on an immediate, spontaneous personal response to the people I've encountered."

Public consciousness of an issue can evolve into a public conscience, maintains James, and it is photojournalism's ability to bring about this shift that makes it such a powerful and valuable vehicle for social change. Indeed, images from the Vietnam War and the American civil rights movement helped to turn the tide of public opinion, he infers. "Politicians and military leaders were telling people one thing. The photographers were showing us something very different. I believed the photographers, and so did millions of others."

Individual photojournalists have an important role to play in creating a body of information and "truth" that can make a difference, he adds. "Journalists often put their lives on the line because they believe that an aware population is essential, that people's voices must be heard, that society cannot function properly without the information they provide or without the stories they tell.

Somewhat sceptical of social media, James is at pains to draw a distinction between 'the media' – the press – and the former. "The press is obligated by a code of professional ethics to report facts as truthfully as possible. Social media is under no such obligation." At its best, however, "social media has a fresh point of view and creativity", he acknowledges. "It's a forum for debate and a cross-fertilisation of ideas that can help unite us."

Convinced by photography's ability to tell intimate human stories powerfully and with compassion in conflict and disaster zones, James believes that photojournalists "record the first draft of history". Photographers are historians, he says, "but they're not looking backwards to analyse the past; they work right on the edge of time itself as the present continuously unfolds into the future."

Photography can also, he muses, "humanise issues that might otherwise appear abstract or ideological or a matter of statistics," while "photographs have the power to make human connections across time and space and to allow a fleeting moment to live on in our collective memory."

Speaking candidly about his experiences of photographing grief, James admits it is the hardest thing to do and only works when people want the photographer to be there, to show their side of the story and reclaim some of the balance of power. "It can only be done with sympathy, respect and humility," he says. "Many of the people I photographed were exploited and victimised. The powers that be tried to silence them, to render them invisible. But a photographer focuses attention on their story."

In this way, photography can provide "a means of reaching out to the rest of the world, even in moments of profound personal grief and sadness, as if to say, this cruelty, this suffering, this injustice, happened here, in this place, to us."

There is an imperative to respect his subjects' dignity, says James, who will show a person's power and strength even in extreme circumstances, such as that of a starving woman he photographed during Somalia's 1992 famine. The skeletal woman, too weak to stand, lies in a hand cart but proudly raises her head and her eyes to meet the viewer's gaze, compelling you to connect with her. "This woman had virtually nothing left except her will to live. As frail and emaciated as she was, she refused to surrender life. Even in the face of all the tragedy and loss she must have suffered, she had not given up hope. Why should anyone else give up hope for her?"

Bearing witness to such suffering is not easy, but James does what he does because he fervently believes in the importance of his work. "What sees me through it is a sense of purpose. It's not even idealism. It's just empirical observation at the effect that journalism – and visual journalism – can have on public opinion, and how it can help to empower change."

Asked whether the longer he spends in a place – or if visiting the same place multiple times – leads to greater or better understanding, he is emphatic. "Absolutely. The more time spent, the greater the understanding. The greater your knowledge of the complexities."

So what impels him to keep going back? "It requires never-ending engagement," he replies. "One crisis is replaced by another. One conflict ends, another begins. Photography has a powerful, positive effect on change. It never happens as quickly as we would like, but it does happen."

"Will photography continue to exist? Absolutely," he concludes. "I think photography is as powerful now as it has ever been."