A picture speaks a thousand words but Brian Harris will be adding a few of his own when he visits Richmond and Twickenham Photographic Society next week.

The international photojournalist, who documented some of the 20th century's most historic events for The Times and The Independent, will be giving a talk on his methods against a backdrop of his best shots.

His first piece of advice? "Before you press the button on your camera, make sure you know why you are pressing that button," says Harris. "You don't need to spend thousands of pounds on a camera. How you take a photo is not half as important as why."

By way of proof, National Geographic has just printed a picture that Harris took, not on a swish digital SLR, but on his young son's happy snapper.

The image, part of a wider project entitled Kiss, shows a couple embracing high up in the Swiss Alps where the Harris family were holidaying a few years ago.

"I saw the moment but did not have my camera," recalls Harris. "My son had his £45 job hanging round his neck, so I grabbed it and my agency were able to sell the shot. I got the sale report today!"

Harris served his apprenticeship in the mid-70s on local papers in east London. Hackney was a hard news area but some of his best shots came from 6am walks along Regents Canal or afternoon strolls in the park, capturing an area, its people, and how they change.

He applied the same approach to photographing the aftermath of war in Rhodesia and the Falklands, the famines in Ethiopia and the Sudan and his award-winning coverage of the fall of communism in Eastern Europe.

Says Harris: "My favourite kind of photo comes from walking the streets of a city - whether it's London, Paris or Norwich. And I still do those pictures. Recently there was this fantastic thick fog in Cambridge where I live, so I went straight into the town and photographed the architecture surrounded in mist, for no reason other than it existed. And those photos will sell."

Harris prefers to see himself as a journalist who happens to use a camera than a photographer-for-hire. As he puts it: "You can have 800 words of perfect prose and a headline but the story falls apart if you haven't got a decent picture to hold it up."

But the profession has changed inexorably in recent years, partly due to the culture of paparazzi, and, more recently, counter-terrorism security measures. Only yesterday, the police sent out a memo to photographers with instructions not to photograph major British railway termini.

And it's not just the authorities. A middle-aged man with a camera faces suspicion from the public, too.

"All this mitigates against the free flow of pictures," says Harris. "But the key is to do your research, to talk to people and not to leave a bad smell behind.

"It's difficult with these big digital cameras not to be intrusive but I try to work with the least amount of equipment possible and I haven't used a flash in 20 years. I would rather miss something and try harder next time than recreate a truth that doesn't exist."