The Brazilian photographer Sebastiao Salgado is not a man to use a handful of photographs when a wall-ful suits him better. Nor is he an opportunistic snapper of intimate moments in Parisian cafes, of nicely dressed bourgeois couples meeting in Trafalgar Square, of over-groomed tots blissfully clutching a collapsing ice-cream cone. For Salgado, photography is, quite literally, the big picture - and the big picture is always based on the big idea. If this sounds hostile, it isn't meant to. Salgado is a man fired with the impassioned commitment of a John Pilger, armed with a Leica rather than a pen.
"It is not surprising . . . that I should identify, even feel a certain complicity, with exiles, migrants, people shaping new lives for themselves far from their birthplaces," Salgado writes in his introduction to his latest collection, "Migrations". "The Salvadoran waiter in a Los Angeles restaurant, the Pakistani shopkeeper in the north of England, the Senegalese hard hat on a Paris construction site, all deserve our respect: each has travelled an extraordinary physical and personal journey to reach where he is, each is contributing to the reorganisation of humankind, each is implicitly part of our story."
"Migrations" is the successor to Salgado's 1993 exhibition (and accompanying book) "Workers: an archaeology of the industrial age". That was at once a celebration of and a lament for the role of the manual worker in a so-called post-industrial society. No one who saw it will ever forget the photographs of the gold miners of the Serra Pelada in Brazil.
In large prints of noble quality, he showed hundreds, maybe thousands, of mud-coated workers humping sacks of gold-bearing soil and alluvial mud up primitive, rickety ladders and across precarious walkways, in an activity not far removed from actual physical torture. Eyes are popping, muscles bulging, faces distorted by the very opposite of the dignity of labour. These men are slaves, and they remind one of the sulphur-mine scenes at the beginning of Stanley Kubrick's Spartacus. There, the slaves, safely shackled, were watched by guards armed only with whips. In the Serra Pelada, the workers are shackled by hunger and the need to feed their families; the guards are in military uniform and armed with rifles, not unlike American prison guards in a chain-gang film.
In the six years since "Workers", Salgado has devoted his endless travels and his questioning mind to the separate, but intimately related, subject of migration. Workers and migration are simply two sides of the same bleak coin, yet there is little sense of duplication in the magnificent exhibition of around 300 black-and-white photographs in the upper rooms of the New Art Gallery Walsall.
One room is dominated by row upon row, hung precisely flush, of quasi-formal, single portraits of anonymous children of all races, from all over the world, united only in their poverty. Many of the pictures contain their own little time bombs: an Angolan boy clutches a football made of rags bound with twine; a child at school in a Sudanese refugee camp has a torso disfigured by burns; another Angolan youngster fails to conceal in a short sleeve the absence of an arm.
These are far from being horror pictures; there is nothing either ghoulish or voyeuristic about them. The children are, even if maimed or hungry, almost all beautiful - wherein lies the real horror of the situation, as Salgado well knows.
His is not, however, merely a sombre vision. There is a strong humanistic, even poetic, side to him. While there are other photographers (Don McCullin, for example) who can select a telling image to make you gasp or even weep, Salgado is the undoubted master of the crowd. Whether it is a pile of Andean women in their black hats or a phalanx of massed agricultural workers carrying their hoes and waving flags and banners, he has an uncanny eye for the impact of humanity en masse.
Normally a hard-edged photographer, he will occasionally surprise by deliberately blurring a shot, as in the white-clad crowd of passengers at Bombay's central station in the rush hour, where keeping his shutter open for a long time created a sense of movement as well as crush, of journeying as well as chaos. This vision of overcrowding makes Waterloo at 9am look like a fete champetre attended by a handful of Renoir beauties.
Salgado has a penchant for the teeming cities constantly in migratory flux, such as Manila, Istanbul and Shanghai. Although based in Paris, he eschews the facile glamour of that city and others like it, such as Rome. Migration is too serious a subject for that kind of superficiality. Perhaps the most striking of his crowd scenes is that in Jakarta's Istiqlal Mosque, one of the largest in the world. Its ground floor alone can hold 10,000 people. I lost count of the endless rows of white-shirted men bowed over in worship. It would take the Elias Canetti of Crowds and Power to do justice to that image.
Yet alongside this colossal Salgado, there is Salgado the miniaturist, as in his photograph of Bombay's Mahalaxmi Dhobighat, a vast public laundry, in which he captures an old woman grimacing with effort as she beats a sheet against a stone wall; in a tiny exposure, Salgado has caught the expelled water releasing a comet-like trail of droplets. Salgado is a unique combination of photographer as prophet and preacher, and artist as technical virtuoso.
In the 1950s, there was a celebrated travelling photographic exhibition called "The Family of Man". It had nothing logistically in common with "Migrations", being an anthology of work by dozens of photographers over a much longer period. Seen with the hindsight of today's eyes, supped full of horrors, it was a sentimental adventure. Yet "Migrations", the work of a single photographer over a finite period, is another, more painful aspect of the family of man, no matter how dysfunctional that family is. "Can we claim 'compassion fatigue' when we show no sign of consumption fatigue?" Salgado asks at the end of his introductory essay. "Are we to do nothing in the face of the steady deterioration of our habitat, whether in cities or in nature? Are we to remain indifferent as the values of rich and poor countries alike deepen the divisions in our societies? We cannot."
Salgado is, with all the overwhelming evidence of his hundreds of great photographs, devastatingly right. As a photographer, he is a major artist; as a social commentator, he is irrefutable.
"Migrations" is at the New Art Gallery Walsall (01922 654 400) until 10 June