'Constraints? What constraints?'

Fred Wiseman is the very model of the modern documentary film-maker. Practically every year since 1967 there's been a new Wiseman film. Usually it is a meticulously observed picture of an American institution or place, shot without special lighting and presented without commentary, interviews or added music. Seemingly without artifice, a Wiseman film apparently offers evidence of what happens before the lens, untouched by the film-makers. His is a puritan cinema as perhaps befits a man who has never left Boston.

Fred Wiseman is the very model of the modern documentary film-maker. Practically every year since 1967 there's been a new Wiseman film. Usually it is a meticulously observed picture of an American institution or place, shot without special lighting and presented without commentary, interviews or added music. Seemingly without artifice, a Wiseman film apparently offers evidence of what happens before the lens, untouched by the film-makers. His is a puritan cinema as perhaps befits a man who has never left Boston.

Wiseman is a unique figure, not because he is a documentary innovator - he started half a decade too late to have pioneered today's dominant fly-on-the-wall style - but simply because of the number of films he has made. Thanks largely to the carte blanche given him in the early decades by the main New York public television station, he has created an enormous movie mural of American life in the last third of the 20th century.

As director, sound-recordist and editor, he - and his camera operators - have been flies-on-the-wall in the high-school and the monastery, the hospital and the army camp, the photographer's studio and the courtroom, the lab and the zoo. There is nothing systematic about this coverage. Nevertheless, the US films achieve an odd coherence, especially as the initial focus on the underprivileged widened to include the wealthy shoppers of the Neiman Marcus emporium in The Store (1983), the yuppies of Aspen (1991) or dancers in Ballet (1995).

Not for Wiseman the normal frustrating business of pitching ideas to sensation-seeking commissioning editors, endless compromises and lack of resources. As a film-maker, he has lived in a world apart. (When once asked about everyday production constraints, he replied: "Constraints? What constraints?")

In the 1970s and 1980s he became a symbol of the value of public television in America, providing the sort of serious fare that not even the new specialised cable channels could afford, much less the old commercial networks.

Wiseman was a Boston law professor who made his first documentary, Titticut Follies, in 1967, about the Massachusetts prison asylum, for the edification of his students. So controversial was the result that the film was banned for general distribution for years. The two main causes of complaint about it have arisen time and again, not just with Wiseman but with most practitioners in the school of Direct Cinema (or Cin??ma v??rit??) to which he belongs.

There is the problem of exploitation. Wiseman has never yielded control as a film-maker and dismisses the rights of those he films as a potential source of "censorship" which he vigorously rejects. This is fine when he is exposing a brutal prison regime but is less acceptable if the person he puts on the screen is more innocent but equally compromised because, say, they are feckless or not too bright.

In Tittitcut Follies he was also accused of selectivity. In general, complaints along these lines are more often levelled at documentaries than are charges of outright fakery. Wiseman, who is unimpeachable when it comes to matters of veracity, cannot avoid accusations of partiality, not even on the grounds that he has not had enough space in the final film to cover the subject thoroughly. This is because, over the years, his films have become longer and longer.Although none match the six hours of Near Death (1989) shot in an intensive-care unit, two hours is a Wiseman short and three hours not uncommon.

Nor would Wiseman use the time-constraint defence if it were available. He is, after all, an artist whose works are his impressions of the world he inhabits. The films are only apparently without artifice since they are most skilfully and cunningly shot and edited, and it is exactly his partialities which make him a documentarist rather than, say, a broadcast journalist.

This is a distinction we seem to have forgotten with our current "factual programming" catch-all. There are now profound confusions as to what documentary means in practice and the essential contradiction of a Fred Wiseman "fly-on-the-wall" versus film artist cannot be untangled. Wiseman himself has not helped with his talk over the years of the "bullshit" of objectivity while claiming he is offering nothing but evidence. It is at best unhelpful and at worst tendentious to claim, as he has done, that "my films are a fair reflection of the experience of making them".

Offering personal vision in the guise of objective evidence is an uneasy combination but it is what makes Wiseman's work so illuminating. His films are more enlightening than straight reportage and more insightful than most fiction. His long march through America's institutions is a wonderful illustration of why documentary is different from any other form of film - the sort of subtle blend of public evidence and private vision that current rows about documentary do not begin to comprehend.

A season of Wiseman's films continues at the NFT this month (020 7928 3232); Wiseman appears at the Sheffield International Documentary Festival, 22 October (0114 275 7727)