The snapper: "Bill Cunningham: New York"

A still-sprightly octogenarian, Bill Cunningham is one of New York's foremost trendspotters. Richard Press's affectionate documentary portrait Bill Cunningham: New York finds Cunningham freewheeling around Manhattan on his trusty bicycle, snapping photographs of those patterns in fashion he spies on the backs, or heads, or legs, or feet of the city's residents for inclusion in his regular "On the Street" column in each Sunday's New York Times. With my usual blindspot for fashion, I'm not entirely sure what effect these collages are intended to have on the reader: are we supposed to look at this pageful of people sporting, say, leopardskin coats and bags and think "Coo, I must get hold of some leopardskin myself"? Or is it more "Cor, everybody's wearing leopardskin these days - best pull out the old tweed instead?" I guess it's a guide, of sorts.

Produced in association with the Times itself, Press's film is a generally uncritical tribute to Cunningham's life and work that could easily be screened at any eventual retirement party. Oft observed bickering with his young male editorial assistant - whose long hair and fondness for T-shirts leave him unlikely to inspire Cunningham to reach for his Leica - the veteran photographer makes for genial company, and Press is keen to celebrate him as a democratiser of fashion: someone who goes out at street-level to find real women who've made (and presumably paid for) their own wardrobe choices, rather than been styled in somebody else's hideously expensive daywear. (We might also credit Cunningham as a democratising influence on fashion photography: the countless blogs and DIY websites that delight in showing what Joe or Joanna Public is wearing surely owe a great debt to the veteran lensman's output.)

There's an admirable purity to the Cunningham approach: he refuses to join the massed ranks of paps snapping celebs at Paris Fashion Week, because he doesn't know who these stars are, and doesn't much like what they wear. Yet despite his motto ("it's not work, it's pleasure"), he comes across as a peculiarly solitary creature, his interest in clothes so all-consuming that, as a poignant Q&A late on in the film reveals, he's never really allowed himself the chance to get close to the people wearing them. That strictly utilitarian part of me did wonder whether what Cunningham is doing - this literally modish pursuit - is entirely essential, or essential enough to merit a feature-length documentary, at any rate: the claims made on his behalf as a fashion historian are immediately undermined by the thought future generations are unlikely to need to know all that much about lo-slung denim, for instance.

Press also has a way of seeking out testimony from the usual look-at-me eccentrics, dandies and self-described "boulevardiers" who tend to set my teeth on edge, and whose flamboyance threatens to overshadow the good-natured modesty and self-effacement of the real subject of the film. At their most illuminating, though, there's no denying both the photographs and the film have a rare and cherishable eye for the hustle and buzz of New York streets, and the colours and artistry these ever-changing catwalks throw up. "He photographs life," one interviewee says of Cunningham - and certainly there's an immediacy and energy in his work one would be hard-pushed to find in any number of carefully lit, precisely choreographed (and doubtless swiftly Photoshopped) magazine shoots featuring the fashion world's deathly skeletal models.

Bill Cunningham: New York opens in selected cinemas from Friday.