In photography everything and nothing is real.

That's the paradox. Photographs capture the world around them, but also recompose, reframe and sometimes even reshape it. You can see the truth (and fiction) of that on show in London and Dundee this autumn. At the Barbican the brutal reality of apartheid is nakedly on display in black and white, while in Somerset House and the McManus the fantasy of fashion shimmers in full colour. Through the camera's lens, everything is possible. Even if it's just capturing the limits of possibility.

That's what's on show in the Barbican's wide-ranging survey of photography in the 1960s and 1970s. This is an exhibition about the politics of the time, perhaps best summed up in one simple image taken by David Goldblatt of a newspaper lying behind a black maid's bed in South Africa. The bed is supported by paint cans. The carpet is flattened cardboard. It is 1969. The headline hails the "moon men on their way back". It's acutely apparent that the distance between first world and third world is just as far as that between the Earth and its satellite.

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Goldblatt is the first photographer you encounter at the Barbican. His images are plain and unemotional but tell the story. His pictures have a detachment when compared to those of his fellow black South African photographer Ernest Cole, whose work was thought lost for years. Recategorised as "coloured", Cole was able to get greater access than many of his black countrymen and as a result his images have an angry immediacy in their attempt to describe the "quality of repression". That repression was everywhere, even down to where you sat. There's a picture of a "whites only" bench in a Johannesburg park. There were no "blacks only" benches.

Bruce Davidson could say the same of the southern US in the 1960s. Like Cole, his coverage of the US civil rights marches is powered by anger at what he saw, whether that be Klansmen on parade or the car of a murdered civil rights activist, the windows shattered, the seats smeared with blood.

But these, like Larry Burrows's huge, immersive colour images of the Vietnam war, are the sixties stories we already know. The exhibition goes beyond that to Mao-era China (courtesy of photographer Li Zhensheng) and 1970s' Mexico where Graciela Iturbide saw the surrealism of native life and framed it with a modernist eye. There's even joy to be found in Malick Sidibe's exuberant studio portraits of the young men and women in newly independent Mali, full of the energy of youth, pop music and the freshness of a new country.

It's impossible to imagine two exhibitions more polarised than the photojournalism at the Barbican and Tim Walker's dreamy imagery at Somerset House. "When you're a fashion photographer everything is contrived from the start," Walker has said." Nothing is real. So what you're trying to do in this fake world is to make a real moment happen by installing genuineness into the artifice."

What's striking is Walker's devotion to his fake world and the lengths he'll go to make it appear. So on display are life-size second world war fighter planes made of bread and giant bees playing the violin – the props that help him create the giddy Arthur Ransome-in-Vogue visions of the past decade. There are also the images themselves: glorious, gossamer-light confections that belie the effort that went into them. Walker has no interest in capturing what's out there. He's out to get at what's inside his own head. That's an art in itself.

But then as Carmel Snow, one-time editor of Harper's Bazaar, told the fashion photographer Lillian Bassman: "You are not here to make art. You are here to show buttons and bows." Shows what editors know. Pictures of buttons and bows are for your mum's old Grattan catalogues. Fashion photography should aspire to something more. But should they aspire to art? That's not a question asked by the exhibition, Selling Dreams, at the McManus Gallery in Dundee, a result of the city's ongoing relationship with the V&A.

What we have here is principally celebratory, a collection of gorgeous imagery, tracing fashion photography from the early years of the last century to the acid colours of Miles Aldridge from the day before yesterday. It includes photographers such as Baron George Hoyningen-Huene (whose crisp black and white image of the model Lee Miller is a knockout), Horst P Horst (represented by his famous 1939 image of the Mainbocher corset later sampled by Madonna for the Vogue video) and Helmut Newton. All are worth a second look.

What a smartly curated show this is, one that rattles through the shifts in style, fashion and grain and yet still finds overlaps years apart. Jim Lee's 1969 image of a model in an Ossie Clark dress complete with background biplane seems like an ancestor of Walker's fantasy approach to fashion.

At its best fashion photography creates its own context, even when it takes fashion out into the streets, a trick it's been doing regularly since the sixties. The American photographer William Klein was a pioneer of that impulse but what's striking in the huge exhibition of his work at Tate Modern is how pedestrian his fashion images are compared to the street scenes he caught on the hoof. Klein is the New York photographer par excellence. His photographs are noisy, messy, busy and in your face. They throb with energy, almost literally in the case of one vivid image of a boy kneeling in front of a chequered-tile shopfront.

However, Klein's images constitute only half the exhibition. You move through New York and Europe to Tokyo and the images of Daido Morayama. The Japanese photographer was influenced by his American predecessor, but has always pursued a more insular, mysterious vision. His are urban images too – "I can't photograph anything without a city," he once said – but his city is dirtier and grainier and at times almost hallucinatory.

That's why he is one of the great contemporary photographers. His keynote images – the wolfish, spooked stray dog and the close-up images of his girlfriend's legs in fishnet tights, a voyeuristic image transformed into abstraction by his lens – are both real and unreal. In Morayama's pictures, the world and it shadow are both on show.

Everything Was Moving: Photography from the 60s and 70s, The Barbican, London, until January 13. Selling Dreams: One Hundred Years of Fashion Photography, The McManus: Dundee's Art Gallery and Museum, until January 6. Tim Walker: Story Teller, Somerset House, London, until January 27. William Klein + Daido Moriyama, Tate Modern, London, until January 20