Even if you have never heard of James Nachtwey, the award-winning photojournalist who is the subject of Christian Frei's new documentary ''War Photographer,'' it is likely that you are familiar with his work. For more than two decades Mr. Nachtwey has traveled to places in the world devastated by war, famine and poverty and documented the cruelty and suffering he has found with an devastating, eloquent clarity. He was in Nicaragua at the height of the contra war, in South Africa during the bloody mid-1980's and in Rwanda in the aftermath of the 1994 genocide.

Mr. Frei shows video clips from those places, and many of Mr. Nachtwey's memorable pictures, some of which are all the more haunting for suggesting, rather than showing, the extent of the cruelty and suffering he has seen. The most terrible image from Rwanda may be one in which neither killers nor victims appear, but one whose frame is filled by a pile of machetes. The film is less a retrospective than a profile of the photographer in action. It begins in the eerie silence of Kosovo in 1999 with Mr. Natchtwey turning his camera on burning farmhouses, grieving families and grave sites and follows him into the poorest sections of Jakarta, where homeless families live beside railroad tracks, and to the West Bank city of Ramallah in the early months of the current intifada.

In some ways Mr. Frei's portrait is exceptionally intimate, allowing us almost literally to see the world through Mr. Nachtwey's eyes. Much of ''War Photographer,'' which opens today at Film Forum, was recorded by a tiny video camera fastened to the body of Mr. Nachtwey's still camera, putting the audience somewhere near his right ear with an excellent view of his busy right index finger. This startling effect of immediacy is necessarily accompanied by a sense of detachment, not only from the people and objects Mr. Nachtwey sees, but from the man himself. On camera Mr. Nachtwey reflects soberly and thoughtfully about his career, and he comes across as a man of deep seriousness and even deeper reserve. Thin and soft-spoken, he has the manner of an ascetic who has subsumed all his ego and passion into his morally and physically demanding work. Following him into the field, we are at a double remove, witnesses, as it were, to his witnessing.

The paradox of being immersed in the horrors of war and deprivation while at the same time remaining outside them, is central to the work he does. Mr. Frei's documentary begins with a well-known quote from Robert Capa: ''If your pictures aren't good enough, you're not close enough.'' Mr. Nachtwey, choking on tear gas in Ramallah and on sulfur fumes at an Indonesian mine, helping a fatally wounded colleague in South Africa or following Rwandan Hutus into the refugee camps of what was then Zaire, could hardly be closer to the action. And yet as he himself observes, he must also remain an outsider, a sympathetic observer of what is happening to other people.

This sympathy may be what distinguishes Mr. Nachtwey from many of his colleagues. He acknowledges that recording grief, injury, death and distress is potentially a form of exploitation, but he makes it clear that the alternative -- allowing man-made misery to remain invisible beyond the reach of those whose consciences should be shocked by it -- is worse.

Several friends -- including the CNN correspondent Christiane Amanpour and Hans-Hermann Klare, the foreign editor of the German magazine Stern -- attest to his immunity to the cynicism that is, like the risk of death, disease or injury, one of the inherent dangers of his profession. Mr. Natchtwey has, for most of his working life, exposed himself to the very worst of humanity and at the same time retained an almost idealistic sense of purpose, based on his faith that documenting war is a small, partial but indispensable step toward its eventual eradication. Mr. Frei's quiet, engrossing film is a sad and stirring testimony to this vision and to the quiet, self-effacing heroism with which Mr. Nachtwey has pursued it.