THE CRITICS: EXHIBITIONS: Realism doesn't come much dirtier

Larry Clark Photology, London Substance Dazed and Confused, London

It seems unlikely that Larry Clark is held in great affection by the tourist office of the State of Oklahoma. In 1971, Clark published a book of photographs called Tulsa, after his Oklahoman home town. Until Clark, Tulsa had been both literally and metaphorically the heart of Middle America. Clark's Tulsa, though, was a different proposition. In 1959, the 16-year-old tyro photographer started injecting himself with amphetamine. Between 1963 and 1970, Clark took pictures of his Tulsan friends doing the same: damaged adolescents in slummy rooms, with ligatures on their arms and hypodermics in their veins. While America comforted itself with the Dick Van Dyke Show, Clark recorded a darker reality at the country's heart.

Time and over-familiarity with photographic horror has done nothing to soften the pictures' hard edge. How so? For a work to remain as raw as Tulsa, it must be sensitive to the rules it is breaking: it must be not merely shocking, but potentially shocked. The curious thing about Clark's photographs is how well-mannered they are. For all their maker's reputation as the high priest of hyper-realism, these are not the creations of some out-of-control speed-freak. Try to deduce Clark from the formal structure of his pictures, and you would end up with a young man who wiped his feet before coming through your door and probably addressed you as Sir.

Take his picture of a young woman, carefully beehived and naked but for a choker, lying on an unmade bed while a bearded junkie injects her arm. The power of the image lies in the tension between two things. First, there is its narrative form: a story of degradation, of possible sexual threat, but one that is told with no obvious moral voice. In narrative terms, Tulsa is simply a catalogue of drug-taking, a neutral taxonomy of amphetamine addiction. The picture's formal qualities tell a different story, though. With her coiffe and her jewellery and her vulnerable breasts, the woman in Clark's photograph is an iconographic type: think of Manet's Olympia.

Or look at Clark's picture of a naked pregnant woman injecting herself with amphetamine. As a narrative, the picture is morally deadpan. Again, it is the formal structure of the picture that introduces the element of moral anxiety. The pyramidal composition of the woman, her pregnant stomach silhouetted in the glow of an unreadable light source, is a form that has been used to denote maternal purity in everything from Raphael's Madonnas to Whistler's portrait of his mother. Whether Clark knew this or merely intuited it, the fact is that there is some kind of internal dialogue going on in his work; an argument between what his pictures are and what they purport to be.

This is not to romanticise Clark's work. The inarticulate tension that makes Tulsa edgy seems to be absent from later works like his knowingly egregious film, Kids (1995). It is also lacking in much of the photography that can loosely be described as school of Clark. Cross London to Old Street and you enter the epicentre of New British cool, in the very middle of which you will find the fashion magazine, Dazed and Confused. In its foyer is a show of photographs called "Substance", curated by the reputed pictorial genius of D&C, the mononominal Rankin. Given the awe in which Rankin is held on magazine picture desks, we might see "Substance" as a small slice of what is best in British photography at the moment: an observation that may make you think wistfully of life in Tulsa.

The trouble with the participants in "Substance" is the glibness of their work. These are not, by and large, pictures about things but pictures about pictures. They have the air of having been made by people who have been to art school and probably know who Foucault is: a stylish closed loop, but a closed one none the less. Gordon MacDonald's close-ups of dermatological glitches - psoriasis, dandruff, cold sores - play little games with abstraction. Roughly speaking, you read them as a series of abstract studies; you realise that they are pictures of athlete's foot or a bruised knuckle; you feel a momentary revulsion at having had your oeil tromped.

Florian Jaenicke's picture of two under-aged smokers does at least not set out to fool anyone. It is just that: a picture of two under-aged smokers, the kind of thing that fans of Dazed and Confused hold to be a new voice in photography, pioneered by Rankin himself. According to this view, the genius of the "new voice" lies in its nihilism, in its having taken the potentially pejorative associations of heroin chic and excised them to leave no associations at all. It is, ironically enough, the same genius that has long (and wrongly) been ascribed to Larry Clark, and easily confusable with vacuousness.

Larry Clark: Photology, WC2 (0171 836 8600) to 4 September; Substance, Dazed and Confused, EC1 (0171 336 0766) to 30 July