There is an image so astonishing in this epochal exhibition you can hardly tear your eyes away. It is a self-portrait by the great South African photographer Zanele Muholi. The non-binary artist appears in profile like the head on a medal, lips white, skin black, against a grainy monochrome ground. Their hair flares upwards in flames so bright they appear virtually silver in the darkness. Or so it seems at first. Perhaps the effect is more like a towering crown, a luminous diadem or a highly patterned headdress. The cultural overtones keep on running, all the way from ancient statues to vorticist painting. But the moment you notice that this hair is in fact a complicated arrangement of afro combs, the political nuances start to ramify. The self-portrait deepens as you look. Muholi was born in Durban in 1972. Their self-portraits have extraordinary graphic force, increasing the contrast so that the artist appears stunningly black in all their regal strength and stoicism. Here is Muholi got up as a black-and-white minstrel, a tribesperson with coils of sinister rope nooses for hair, or with fuse wire around their neck. In one image, only the whites of the eyes are visible; in another, the artist appears in a miner’s helmet, with the implication that they have just risen up from the darkest depths of the earth, bearing the coal dust of their labour. This is an act of solidarity, an honouring of the 34 striking miners murdered by South African police at the Marikana mine in 2012. Muholi has also appeared in necklaces of tyres, and wearing a wooden stool on their head in sardonic pastiche of western ideas of darkest Africa. A tremendously strong self-portrait at Tate Modern looks like an ethnographic photograph taken by some Victorian explorer. It shows the artist’s head bristling with what might be bones or sticks, but are in fact pens. This double take is as mordant as the work’s title, Nolwazi, which translates as “knowledge”. The photograph refers to the dehumanising “pencil test” practice used by the South African government in racial classification under apartheid. If authorities weren’t sure whether someone was truly white, a pencil would be inserted in their hair. If it dropped out, the person “passed” as white. It would be an understatement to say these images make you think twice about race, colour, imperialist oppression, state cruelties – historic and continuous – of all kinds. Just to stand before any of the self-portraits in this lifetime survey is to be confronted by images of exceptional beauty – exquisitely lit, brilliantly conceived, in all their profound intelligence – yet never to be lost in simple admiration. This is an art of agency, meant to stir; this is portraiture as activism.