"The best fashion show is on the street. Always has been, always will be." So says Bill Cunningham, 82-year-old godfather of street style photography, in a new documentary about his life and work.

As London fashion week opens, the once-rigid boundaries between catwalk and street fashion are increasingly blurred. Celebrity bloggers such as the Sartorialist now get VIP seats at catwalk shows. There are now as many photographers documenting the fashion show audience – and amateur photographer-bloggers in the audience documenting each other – as photographers stationed in the traditional end-of-runway "pit" to capture the catwalk looks.

So it is fitting that the most talked-about film in the front row is a documentary about a photographer who has a front-row seat at every major fashion show but whose passion is to shoot interestingly dressed people on the streets of New York.

Bill Cunningham's taste is revered by Vogue's editor, Anna Wintour – "we all get dressed for Bill," she says in the film – but he dresses each day in the utilitarian blue jackets worn by French street-sweepers, which he buys in bulk from department stores while visiting Paris fashion week.

His pioneering career as a photojournalist of street style took off serendipitously in 1978. He was in his regular spot on the corner of Fifth Avenue and 57th Street, taking photos of elegant or unusual outfits for an occasional column in the New York Times, when he spotted a woman hiding behind the turned-up collar of a fur coat. Having been a reporter at Womenswear Daily, Cunningham spotted the classy cut of the coat and took a series of photos. Back at the Times office, it transpired that he had captured a rare sighting of Greta Garbo.

Soon afterwards, Cunningham's On The Street column became a regular feature, as it remains. The New Yorker called it "New York's high-school yearbook, an exuberant, sometimes retrospectively embarrassing chronicle of the way we looked".

Bill Cunningham New York is not a film about fashion. Rather, in tracing the emergence of street style as a prominent facet of popular culture, the documentary explores the link between a newspaper and the street-level culture of the city it serves, and the role of a reporter in mediating the shifting sands of this relationship over the decades. Cunningham is a reporter who travels around Manhattan on a bicycle using his camera to record what he sees "as if it were a pen … There are no short cuts." He says in the film: "You have to get out there, and let the street speak to you. You can't report to the public until you've seen it all."

He documents high-society parties at the smartest Manhattan events, as well as catwalk shows and people on the streets of Manhattan, in order to build a 360-degree profile of fashion. "I just try to be honest and straight," he says.

Cunningham makes it a point of principle never to accept even a glass of water at any event he attends, saying: "I like to keep my distance, and I would never compromise the Times."

His straightforward approach has on occasion stirred up trouble: he has an encyclopaedic knowledge of fashion history and has been known to flag up plagiarism by juxtaposing new catwalk photos with similar looks from the past. In the 1980s, he caused controversy by pointing out the links between the work of Japanese fashion designers working in New York and "the bag women on the streets at that time … [both] had these shapes, which were close to what you might see in medieval Europe."

The film's director, Richard Press, who met Cunningham while working as a graphic designer at the New York Times, has said it was Cunningham's "almost religious dedication to his work" that inspired him to make the film: "Not only a portrait of him and by extension New York, the city he loves, but a celebration of self-expression and self-invention."

The zeitgeist is catching up with the forward-thinking Cunningham's stance on fashion, as the once-exclusive fashion industry opens its doors in response to a swell in interest from the public. Fashion blogs are growing in number and status, while live streaming is transforming access to the catwalk experience.

Harold Koda, curator of the costume institute of New York's Metropolitan Museum, says Cunningham is "a true egalitarian".

"It's not that he isn't aware of cultural division and hierarchies but he treats it all the same."

But even Cunningham – who colleagues say "has never taken an unkind picture" – is not above the occasional snub, albeit unintended. Anna Wintour reveals in the documentary that Cunningham, who has been photographing her "since I was a kid", still frequently documents her outfits: "It's one snap or two snaps. Or sometimes he ignores you, which is death."