For anyone who considers Vogue to be entirely about fashion, and fashion to be entirely an irrelevance, 100 years of evidence to the contrary comes in the form of the National Portrait Gallery’s wonderful exhibition, Vogue 100: A Century of Style.

First, this is not only a show of fashion photography. British Voguehas always been about people, and about powerful people at that, men as much as women. Anyone who is anyone has appeared on its pages; nobodies are usually nowhere to be seen, unless they are a representative of type, as in Irving Penn’s sublime “Small Trades” series of 1951. (Penn’s Covent Garden market porter, who appears — joy of joys — to be looking down his working-class nose at the Voguereader, is an exhibition highlight.)

So there are numerous famous portraits on show here, the most fascinating being of individuals caught in the opening seconds of their five minutes of fame. (“British Vogue has always been more interested in new things, new talent, rather than in people who are established,” its current editor Alexandra Shulman tells me.)

Here is a velvety-looking twentysomething Gore Vidal, for example, and, by way of contrast, Lucian Freud at a similar age, as beady as the pet sparrowhawk in his hands. (The Freud portrait is one of Shulman’s favourite photographs published in the magazine. “He looks like Bob Dylan, and the bird has that same beaky sense of menace.”)

Both men were photographed for Vogue just after the Second World War by Clifford Coffin, one of the magazine’s greats, only comparatively recently rediscovered, and as monstrous as he was brilliant. (“From the rubble of emotion emerged a perfect cool picture,” observed Vogue in 1966. “A weird, wild man,” remembered the model Wilhelmina.) It was Coffin who stepped into the postwar breach when others on the magazine’s photographic roster, such as Cecil Beaton and Lee Miller, were still wrapped up in foreign parts.

Decades later, there’s a portrait of a little-known novelist called Salman Rushdie, also in his twenties, recognisable only by his midnight eyes, and another of an up-and-coming actor called Rupert Everett, nude at a window, at once pre- and postlapsarian in best Another Country-style. Both portraits were taken by Snowdon in the early 1980s.

Then there’s the barely recognisable Uma Thurman aged 15, photographed by Sheila Metzner wearing what looks suspiciously like curtains in 1985. And the now celebrated 1993 shoot by Corinne Day of a “cocky kid from Croydon” in a vest. “She wasn’t like a model,” Day said of Kate Moss later, “but I knew she was going to be famous.” What started out a fashion story has since transmogrified into an intimate portrait of a proto-superstar.

Because of course there is straight fashion photography too, and plenty of it. Some of it — in its detached elevation, its very exquisiteness, its (at times) downright oddity — effortlessly announces its status as art to those few remaining sceptics who still misguidedly believe photographing fashion can only get you so far. There’s Horst’s famous 1939 surrealist portrait of a model in a Detolle corset, for example, limbless like a Greek statue. And Nick Knight’s iconic 1996 close-up of a black pearl caught in the violet-lipped shell of a model’s mouth. There has never been any question that these are images worthy of a space on a gallery wall, as they will have next month.

But then there is the fashion photography that is not detached, that juxtaposes the elevated world of expensive fashion with the earthbound; that offsets fantasy with an encroaching sense of the real. These are images that manage to encompass both fiction — the necessary fiction that sustains the expensive business of luxury fashion — and fact. They may be less self-consciously artful than the work of fashion purists such as Horst, but, to my mind, it is precisely this that makes them the most persuasive images in the exhibition as to the relevance, even dare I say it, the importance, of fashion imagery.

Most notable is Coffin’s ground-breaking 1947 portrait of Wenda Rogerson — Mrs Norman Parkinson — wearing a ballgown in a once grand, now bomb-damaged staircase in Grosvenor Square. Coffin was one of the first to take fashion out of the studio onto the streets — and staircases — of the real world. That shrapnel-pocked, open-to-the-elements flight of stairs-to-nowhere, fragile as lace yet still monumental, enduring, evokes with wonderful sleight of hand the story of Britain during the Blitz years. (Not that Coffin always found the real world suitable for his purposes. Sent to photograph Stonehenge, one bemused witness later recalled, he “felt the stones should have been very much larger than they were. He did one shot, then stormed off.”)

In similar through-the-looking-glass mode there’s Frank Horvat’s 1960 shot of a tweed-clad model in a cobbled Bradford back alley, the polish on her pointy black patent pumps contrasting with the scruffy shoes of the working-class children who surround her, not to mention the lacklustre street itself. And there’s a particularly delightful 1969 park-bench encounter between the French model Mouche and two of Regent Park’s more regular denizens: they are there to feed the pigeons, she to demo the new-season tailoring. All three women, photographed by Barry Lategan, are wearing headscarves, but only one of them has her knees out.

Perhaps surprisingly Helmut Newton was a documentarian too, but only of the glamorous: no benches and backstreets for him. “Although it makes my life more complicated,” he once said, “I prefer to take my camera into places often inaccessible to anyone but the rich.” So we hang out with him, and assorted other beautiful people, at the Hotel Byblos in St Tropez, or the Crillon in Paris, both ground-zero for chic in the 1970s.

There were periods when Vogue missed the boat, of course; when it was too busy weaving its own high-class fictions to notice the story unfolding in the real world. Despite having groovy young things like Terence Donovan and David Bailey on its books, for example, it was remarkably slow catching on to the youthquake of the 1960s. (“Bailey felt for years that he would go in there and they would pretty much pat him on the head,” says Robin Muir, the exhibition’s curator. “But he needed the salary. He couldn’t say no.”)

Vogue had launched a circumscribed section within the magazine called “Young Idea” in the mid-1950s. Eventually it was to realise that there was no other idea, not in fashion at least. (The fashion industry is still trying to disentangle itself from this one.) And so Beatrix Miller was poached from the far hipper Queen magazine to be editor, and Vogue finally started Swinging with the rest of London.

From the magazine’s earliest days, being signed as a Vogue staff photographer was the best gig in town, its monthly stipend a kind of patronage, the quality of photographic reproduction nowhere else to be found. Which is in part why there are so many photographers in the exhibition you perhaps wouldn’t associate with fashion: Edward Steichen, Man Ray, Charles Sheeler. As Muir observes: “Vogue’s photographers are not just the great names of fashion, they are the great names of photography full stop.”

Even those who considered fashion photography beneath them — most notably Steichen and Sheeler — couldn’t say no. “You were contracted to do fashion, and an awful lot of fashion,” continues Muir. “But you also got to do still life and portraiture. And you got all the best models. Jean Harlow, Gloria Swanson, Charlie Chaplin, they all wanted to be in Vogue. Plus there was a great studio, lots of assistants. No other magazine had that kind of infrastructure. Steichen made the trade-off: he had to do the fashion but he got to do the portraits too.”

In the early years many of the biggest photographic names were, like Steichen, inherited from the American title. (British Vogue’s incongruous-seeming launch date of 1916 came as a result of the Atlantic blockade, which stopped copies of the original title reaching London.) Gradually, though, the offshoot title began to pull away.

“Cecil Beaton is perhaps the first true child of British Vogue in terms of photographers,” observes Muir. “He submits his first picture when he is a student at Cambridge in 1924. Very quickly he is their No 1 photographer.” Beaton knew that Vogue would be the making of him. When the editor wrote to him “from her Mount Olympus home”, he later recalled, “I felt I was on the road to fame.” (Beaton’s capacious address book, always a big plus at the unashamedly elitist Condé Nast publishing house, also provided rocket fuel for his meteoric rise.)

Beaton’s remarkable proteanism — “he was everything to them,” says Muir — is in evidence through the exhibition. There are his sharp-as-a-knife portraits, often with waspish written asides. (Jean Cocteau famously called him “Malice in Wonderland”; the Duchess of Windsor, Beaton pointedly observed, “reminds me of the neatest, newest luggage”.) And there are his gloriously grandstanding fashion set-pieces. One can trace a direct line from the latter — the most celebrated being his 1929 “Soap Suds” confection of balloons, cellophane and it-girls-as-“bubbles” — to the fantastical creations of one of Vogue’s current stars, Tim Walker.

Walker’s peculiarly British brand of fairytale — as exemplified by his 2008 portraits of Helena Bonham-Carter in a shattered glass lift and Karen Elson in bed with a crocodile — is one of the visual touchstones in the modern British Vogue, along with the gorgeous razzmatazz of Mario Testino’s celebrity-land, and the beauteous cerebrality of Nick Knight’s semi-abstractions.

“An ongoing dialogue with a photographer is really important for an editor,” says Shulman. “The fact that people like Mario, Nick and Tim contribute regularly to Vogue is key. They are a huge part of the magazine.”

Shulman’s favourite shoots, she tells me, are “when I can persuade a photographer to create something that has to do with an idea outside of fashion — with art, food, travel, film”. It is this ambidexterity, always integral to the magazine, that makes the new exhibition so enchanting. Here is a portrait of a century of fashion, but also of a century of life.

Vogue 100: A Century of Style, sponsored by Leon Max, is at the National Portrait Gallery, WC2 (020 7306 0055), from February 11 to May 22