In a dream she recorded in great detail in one of her tiny notebooks, Arbus is in a splendid hotel as a fire takes hold. In an atmosphere of slow, calm catastrophe likened to the demise of the Titanic, she is both enthralled and in a state of anxiety. She half-expectently feels she should take responsibility for saving people but more intensely feels a growing frustration as she realizes she hasn't got her camera to hand.

Being stopped by a stranger with a camera asking them to pose for a snapshot was met with either a baffled willingness or, alternatively, a silent refusal. I'm not talking about Arbus here, this is when I asked people queuing to enter the Arbus show if they would let me take their picture. In fact most said yes. Several, the wealthier and more distinguished visitors, said, well nothing. A shake of the head and an averted gaze as they moved inside, much as they would avoid a beggar. Diane would have had a better chat-up line, I expect.

Ironically those who declined to be photographed were on their way to look at her photographs of people who had been asked to be photographed in a similar manner. Being accosted by a photographer is not the kind of social encounter people are used to, so what was it about the people who actually chose to say yes? Why did they agree? Those who were amenable seemed to me to be more comfortable in themselves, had perhaps fewer insecurities or suspicions - or maybe they simply felt they had less to lose.

If you're reading this you probably know a fair bit about Arbus's life and you are a fan of her photography. There are plenty out there though who dismiss her as some freak-show uber-bitch ringmeister. Germaine Greer, for one, probably won't be going to the show. And imagine if Arbus sought funding from a US equivalent to our very own Arts Council. Her fetishistic interest in identity and marginalisation come from the wrong side of the postmodernism tracks and her 'notions of gender' et al, simply aren't adequately didactic enough to be safely 'on message'.

Luckily for Arbus she had several supporters in her own time. The Guggenheim stumped up grant money twice in the mid sixties. She also had Alexey Brodovitch, Edward Steichen, Lisette Model, John Sarkowski, Marvin Israel on her side. Not bad! Now it's thanks to Sandra Phillips and Elisabeth Sussman at the San Francisco Museum of Modern Art that this well researched and lovingly curated show has come together in our time. The last big Arbus show was in 1991 at Toronto's Ydessa Hendeles Art Foundation but It is over thirty years since the last major gallery retrospective of the work, which occurred the year after her death. Most of the prints on show have actually been made posthumously by Arbus's friend Neil Selkirk, who was once a student in her class. He stays faithful to her avoidance of interference in the print process which she believed was essential for the credibility of the image. As well as the photographs we get to see her notebooks, some letters, her cameras, the art books she owned, even her darkroom has been installed. In the minimal light it's not that difficult to imagine her walking in.

Martin Barnes at the V&A bagged the show for London as it moved base between Essen and Barca and there has been the inevitable vitriol here. Greer complained on 'The Culture Show' (BBC2) that the image Arbus chose of the 'Jewish giant at home with his parents in the Bronx, N. Y. 1970' was the one cruel shot on the roll. In all the other frames the family are posing together as for a family snapshot. The twelfth and final frame is an observational one taken when the session appears to be over, with the subjects off-guard. The family have separated and turned away from the camera to look hesitantly at each other, to collect themselves. In that transition Arbus's twelfth and final shot extrapolates a moment of minor discomfort into psychological shell shock to electrify the blown-out wide angle frame. We are left to muse on the disaster that has befallen the family. Perhaps this is unpalatable but I wonder if her detractors were to encounter Arbus's milieu in a Wolfe novel or a Velvet Underground album they would feel less distaste and far more acceptance. Crucially in the medium of photography it's not feasible for the artist or the audience to disengage from the subject to such a safe distance, exposure is more immediate and far less mediated. As she went about testing herself she is vicariously testing us and we are face to face with her moral challenge.

Her photography was courageously full-on, in your face and shameless. She rarely shot undercover and also abandoned early experiments in grainy emulsions for gritty detail and expressive tonality. She upgraded from 35mm to a larger two-and-a-quarter inch roll film Rolleiflex and consequently had to endure more technical hassle but achieved more impact. As well as providing her with her trademark square format it employed a waist level finder, advantageous as she preferred shooting without maintaining eye contact, which avoided engaging her subject too much before she got the shot she was hoping for. After that she was just as likely to attach herself to them and keep shooting as they spent some time together. Technically she was not strong, the negs are often poorly exposed and it's amusing to see her typed shutter speed guide taped onto the back of her exhausted Rolleiflex. The photographic result justified the means to Arbus and the search for new, extreme encounters drove her on till she had the insight to conjure it vividly from even the mundane. Her forays into journalism were largely a strategy to support this obsession. Whatever analysis we might choose to explain her motivations is not that important in the end, it is the gripping photographs that remain are what count for everything.

My personal long-time favourite Arbus photo is the breathtaking 'A Child Crying, New Jersey 1967' (link here to online image at George Eastman House Foundation). I first saw this in the Aperture monograph of Arbus's work about 25 years ago and I unexpectedly crumpled before it, silently howling for a few grief stricken minutes. Like much of her greatest work it took artistic courage to take a shot which simultaneously betrays a mean, selfish opportunism. As a human being Arbus found herself beyond redemption and her revelations about what it is to be human were eventually paid for in full. I was glad to see this photograph in this show and it still raised the hairs on the back of my neck in sheer wonderment. An infant, a girl of about four, stands weeping in the New York twilight. Her cardigan is buttoned up to her chin to keep out the cold, wisps of hair shimmer and tears well in her eyes and roll down her round face to her chin. Her skin is soft over a sculptural, stone-like head, a brow crumpled with unhappiness. The mouth is a cavernous black-hole, a frozen slit of pain, and eyebrows arch questioningly at the arrival of Arbus.

If you then come to reflect on it coolly enough you come to realise that Arbus's reflex on seeing this despair was to move in on her, crouch and steady her camera a few feet from the transfixed child's face. She took time to find focus, set the aperture and shutter speed and fire off the shot with a flashbulb to be sure of good sharpness and exposure- which would have been blinding for the subject, particularly from such a close range. The photograph is astonishing, even overwhelming. I guess the best one can say about Diane as a person is that she probably didn't like to hug much.

In the last two years of her life she made images at residential homes for people with mental disabilities. This arouses most contempt in those who like to feel contempt first and don't bother with the asking questions later bit. In my opinion the images are both full of a perky sense of fun and, even more unexpectedly, full of something that feels pretty much like a loving gaze. Arbus thought these pix her best work and was exhilarated making them. Her creative achievement would be significantly diminished without her having made them and no-one who believes in Arbus should feel the need to apologise for her taking them. Yes, even when photographing them in halloween masks and with their faces painted. However uncomfortable one of course feels about these pictures the truth is these are profound and incredibly meaningful images and may indeed be her best work. Arbus refuses to show deference to the mores of mainstream taste, the same taste that hides people deemed unfit to be seen. She instead celebrates that which we call ugly and those we see as monsters. You either trust Arbus here or you listen to the detractors and finally turn your back on her. It's in making us face these difficult choices that makes Arbus one of the major artists of the twentieth century.

Considering some of the less salubrious places where she'd hang out she contracted hepatitis which may well have contributed to her decision to commit suicide in the Summer of 1971. Stupefied on barbiturates and poised to cut her wrists I imagine she would have felt a forlorn part of her wishing to be stood a few feet away photographing the proceedings. She may even have considered setting her Rolleiflex's self-timer to leave behind one final exposure, this time a self-portrait of herself, lost, at the very edge of somewhere dark and infinite.

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Photographs of visitors to the Diane Arbus show at 4pm on 16th Oct '05