The narrative preoccupations of the nine artists are unrelated - though, for the most part, not unphotographic. Bernard Frize, for example, has taken a couple of pictures of clouds; Roni Horn has shot some Icelandic scenes; Rosmarie Trockel has taken, among other things, a picture of a monkey; Isa Genzken has photographed X-rays of her own skull. All traditionally photogenic stuff, but stuff which betrays an uninspiring finality.
Stylistically, the show is more cohesive: in matters of exposure, contrast, focus and tonal range, the works are loose. But it would be a nave curator who did not recognise that self-proclaimed 'photographers' now routinely tamper with the very same parameters, subverting the same so-called norm. And this formal fluidity does not make them any the more interesting.
The two exceptions are the contributions from Sigmar Polke and Gerhard Richter - which are most notable for not being photographs at all. Polke, for example, has merely exchanged canvas for photographic paper, brush for a torch and paints for photo-sensitive chemicals. He has not used a camera, there has been no appropriation from the outside world - he has merely dabbled with abstract strokes of light on photosensitive paper. The results are strange, jerky abstractions, full of tension. The chemicals seem to have an agenda of their own, and the artist's struggle to manipulate them reminds us just how crude the raw ingredients of photography are.
Richter continues his long-running dialogue with photography by obscuring a holiday snap with oils. Depending how you look at them, such snaps are either so standardised and ubiquitous as to be meaningless, effectively offering the painter a blank canvas, or, conversely, something so personal that they need, for privacy's sake, to be disguised by a thick molasses of paint. The paint serves as a warning to the viewer that the picture beneath is of value only to its creator.
But the four images (or anti-images) by Polke and Richter hardly amount to a debate. As for the rest, if they do have a message it is discreet rather than discrete. The question remains as to whether the generic term 'artist' warrants a collation of their work. The answer? Probably not.
The problem confronting any exhibition which assembles work under some unifying banner - whether it be artists or the family ('Who's Looking at the Family?'), or indeed life itself ('All Human Life') - is that the viewer goes in expecting to learn something about that banner, and somehow anticipates that the show itself will ultimately be greater than the sum of its parts.
Often the contrary is true, and this is perhaps particularly true on the contemporary photography circuit. 'A Positive View', which has just finished a run at the Saatchi Gallery, was an example of how such exhibitions, far from being holistic, can negatively implode. It wasn't so much that the images brought together for the exhibition - mainly fashion shots and portraits of the rich and famous - were bad. As you would expect, with 200 photographs by 150 known photographers, there was no shortage of quality. Nor is it that the values embraced in the high church of fashion are often so unpalatable. It isn't even that one wants to protest against the fashion industry's relentless claims to being innovative and culturally responsive, when the evidence clearly says otherwise.
It is rather that, thrown together en masse, the well-groomed, banal images demeaned the more provocative ones. The juxtapositions are too tart to digest. Koo Stark's A Right Tit, for example, is a near neighbour to Serrano's Last Supper; David Sim's picture of Linda Evangelista's now institutionalised pout takes up where Cindy Sherman's B-movie stills series leaves off; Terence Donovan's glance down Cindy Crawford's cleavage nestles against a Hockney collage. Since the works by Hockney, Serrano and Sherman were not for sale, one can only conclude that they were brought in to provide an erudite gloss over the vacuity of much of the Vogue-sponsored material. The result is that we are inclined to assess these works in the context of the rest - our confidence in the self-conscious, self-referential status of these works as 'art' is critically shaken. Thus Cindy Sherman's B-movie stills are whizzed past as fast as the monozygotic Evangelistas - misread as yet more glam shots destined for the coffee tables of the monied few.
It is refreshing, therefore, to see Jo Spence's 'Collaborations' at the Royal Festival Hall. The exhibition needs no generic justification, and stands boldly alone. This is a set of self-portraits (though she credits other people for having actually released the shutter) - Jo Spence in the bath with binoculars; Spence as a cultural sniper (wearing a mask and firing a catapault); Spence as charwoman (scarf, broom and fag hanging out of her mouth); Spence as a baby (with bonnet and dummy). But these self-portraits are anything but conventionally narcissistic. There's nothing of, say, Duane Michals or Jeff Koons in them. Indeed, she portrays herself in an ugly, unsoftened light. Nor is she, by dressing up, trying to mask her own identity as Cindy Sherman did. Rather, Spence's works are the product of a genuine period of reflection and introspection.
The inspiration for the pictures was her imminent death: diagnosed as having terminal breast cancer, she abandoned taking pictures of the outside world and turned the lens on herself. 'I began', she wrote, 'to use the camera to explore connections between myself, my identity, my body, history and memory. I was beginning to inhabit my own history and hidden parts of myself. I took and used photographs to help me ask questions rather than reiterate what I thought I knew already.' She called this process photo-therapy. Spence had a theory that re-enactment could lead to resolution and release. If she could come to terms with her identity she would stand a chance of coming to terms with her own mortality.
Whether this photo-therapy was effective we do not know. We do not know, for example, whether the distress that she had felt during her life about not belonging to a tangible class had been alleviated by her dressing up as her working-class mother. Or whether photographing her breast with her own name imprinted upon it helped her cope with the pain of a mastectomy operation.
What is effective, though, is the way in which these pictures hammer home the lot of the mortal. The viewer is put in the uncomfortable position of voyeur to one person's brutally honest, angry and frightened attempt to come to terms with the senselessness of her fate. Her attempt to take control of her situation makes her impotence the more transparent.
'Discrete Images: Artists' Photographs', Frith Street Gallery (071-494 1550) until 4 Nov. 'Collaborations', Royal Festival Hall (071-921 0600), to 16 Oct