Paul Graham's exhibition at London's Whitechapel gallery is filled with people who are just looking. We observe their rapt attention, their lostness, absorbed in things that we can't see. They stare at TVs beyond the frame, their faces caught in the glow of the screen; they look out of cafe windows, distracted by the passing traffic. They wait in dole offices whose grimness is an insult to the eye. In the corners of nightclubs, people stand with their eyes closed, engulfed in music, or booze, or drugs. In Belfast, a woman squints at the smoke from the cigarette she's dragging on. A pensive Galician girl casts her eyes downwards in Vigo. People stare at the pavement. They gaze at the floor. Wretched walls return their vacant looks. Sometimes they don't seem to see the world at all, their thoughts engaged elsewhere. We see their interiority and distraction, but cannot penetrate it. Sometimes they see nothing because they really are blind. Sometimes we are blinded, too, and the picture goes almost blank, too full of glare for us to see. There's something going on in there, but the detail has been blanched out. This is a whited-out view of America, black neighbourhoods bleached to the point of erasure, just like the affliction visited on everyone in José Saramago's 1995 novel Blindness, in which Graham discovered a telling affinity with his own work. Made between 1998 and 2002, American Night juxtaposes these over-exposed images with over-rich, colour-saturated shots of sturdy homes in affluent suburbs, and often shadowy shots of black people on America's streets. As a photographic essay, American Night is as much conceptual as it is social critique, as perversely poetic as it is observational. It is difficult not to regard all of Graham's projects as metaphor, not least for the photographer and his subject, for engagement itself: looking at photographs and at the world; looking at other people looking, seeing and not seeing. The camera sees more than the photographer – or rather, it sees something different. We see with the mind more than the eye, while the camera itself is only an eye, wherever the photographer directs it. When I look at Graham's photograph of the view from the bridge over the Archway Road at Highgate in north London, I see things from my past that Graham can't possibly know, and which aren't actually in the picture. The past, and your own life, comes tumbling in. The same is true of the wretched pictures he took in employment offices during the early 1980s. I have sat in at least two of these self-same DHSS offices, waiting for my number to be called. Graham waited, too, not only to take sly shots, but to sign on himself. Beyond Caring remains Graham's best-known series; laminated versions of these photographs were toured by the old Greater London Council to TUC conferences in order to lobby MPs for better conditions in these desolate waiting rooms and interview booths. They remain often appalling and dehumanising places, for claimants and staff alike. But there's more to Beyond Caring than social documentary and observation, or even political commitment or outrage against Thatcherism. There's a terrible emptiness in them, a blight that goes beyond the economic. In a larger sense, this is the subject of his series New Europe (1988-1992), showing us often desultory and seemingly insignificant places and moments in Germany, Spain and elsewhere. A one-armed man (how did he lose it?) stares across a wasteland toward the city beyond, this picture paired with a second image that shows the interior of a bleakly anonymous, glass-walled room, lit by a circular fluorescent light. The glass wall reflects shimmering chandeliers (maybe it's the interior of one of Spain's casinos). In another pairing, one shot shows a laughing young woman – perhaps outside a nightclub. This is coupled with the arresting image of fresh globs of spit, spattering Franco's gravestone at the grandiose Valle de los Caídos (the Valley of the Fallen), west of Madrid. Many visitors must have made the journey for the sole satisfaction of spitting on the dictator's grave. The site was closed to the public two years ago. The past and present collide in all these images, though frequently in ways that aren't all that obvious. Looking for trouble Graham's colour photography began, he has said, as a "mash-up" of William Eggleston and Robert Adams. Eggleston, the pioneer of colour photography, of rooms and streets and people, and Adams, primarily a photographer of landscape, are an unlikely pairing. Graham has also been inspired by photographers and artists as diverse as Jeff Wall's constructed and posed photographs, and the elegiac, black-and white (though mostly grey) work of the brilliant Berlin photographer Michael Schmidt. In Graham's work, conceptualism, social critique and the wandering eye of a man who would travel America's roads (like a latter-day Robert Frank) with a camera but nothing much in mind, collide. Odd corners, the insignificant, a guy mowing grass or a man carrying a big carton of Pepsi through a winter day, another guy smoking – these all come from a series of books Graham made called A Shimmer of Possibility. And it is often in books that I like looking at Graham's work best. His photographs, after all, are often meant to be seen as extended essays as much as as individual shots. Graham's exhibition fills three of the Whitechapel's spaces and takes us from 1981 to 2006. Even though it is a large exhibition, it feels incomplete, or perhaps just too brief. It isn't that the photographs are too empty, or lacking in detail – some, like the images he took in early works such as Troubled Land, a tour of Northern Ireland during the conflict, are all about the details. A Union flag waving at the crown of a tree in a field, a distant soldier running across a roundabout in a neat suburbia, the helicopter hovering over the South Armagh hedgerows, the watchtower over the hill. Often, we have to search out the telling incident in the shot. Is it the cows grazing in a distant field? Is it the paint spatters on the road? Where is the republican parade mentioned in the title of this view of Strabane, with the rain coming in over the church spires, the distant bridge over the river? It takes a while to spy the line of tiny figures, almost obscured by the grasses in the foreground. Graham went back to Ireland in April 1994, when the ceasefire had been announced, and just pointed his camera at the sky. The photographs in Ceasefire show nothing but the passing weather. The veiled sun glimmers through cloud in Ballymurphy. More dark clouds lower in Newry, and weather moves in over Shankhill. Like peace itself, these cloud studies are a kind of sublime abstraction. The indifferent weather, we also understand, respects no borders or allegiances, whatever Ian Paisley once said about never forsaking "the blue skies of Ulster for the grey mist of an Irish Republic". It is in the accumulation and juxtaposition of details and moments and runs of images that the evolving narrative of Graham's varied, complex art unfolds. Somehow this show feels just too well-tempered – too edited, too neat – even across several of the Whitechapel's spaces, to let its cumulative effect take hold. Like the world itself, Graham's art is messier than this.