Sing Your Song (12A) *** Woody Allen: A Documentary (E) ***

Harry Belafonte has lived his entire life in the public eye. At the same time he was becoming a worldwide star as a crooner with a lilting and seductive voice, Belafonte was fighting against injustice wherever he saw it. Susanne Rostock's engrossing documentary portrait shows how Belafonte took his cue from an earlier singer/activist Paul Robeson in using his celebrity as a political tool rather than as a means to money and fame. "Get them to sing your song and they'll what to know who you are," Robeson told him.

Belafonte had a tough childhood but he doesn't labour the point. He was born in Harlem, New York, but sent at an early age to live in Jamaica by his impoverished mother. She told him that he should never spend a single day in which he didn't do something to "undermine" injustice.

This is a fascinating but very lop-sided film that largely skims over its subject's private life. Maybe that's the point. Belafonte speaks fervently and in very great detail about his friendship with Martin Luther King, his relationship with JFK and Bobby Kennedy and his admiration for Mandela. When it comes to his own family, he is far more terse and evasive.

He mentions almost in passing that his marriage split up because he wouldn't abandon his social activism and his young bride couldn't help but listen to what his McCarthy-era critics were saying about his "un-American" activities.

If his relationships withered or he wasn't always there for his kids, that was the necessary fall-out. One of his most famous songs, "Scarlet Ribbons", he says as if in apology for ignoring them, was aimed at his children.

What the documentary doesn't even begin to tell us either is how Belafonte emerged as a performer. He describes how, when he was a janitor's assistant, he was inspired by the American Negro Theatre. As a young, would-be actor, Belafonte studied alongside Marlon Brando, Walter Matthau and Tony Curtis. Not that he wants to share any reminiscences about this period in his life or about how he became Broadway's "newest golden boy". As he puts it, he had "bigger concerns". Rostock's documentary fills us with admiration for Belafonte's courage and persistence as a political activist. At the same time, it also feels evasive.

If Belafonte lives entirely in the public eye, Woody Allen (the subject of an equally fascinating and equally frustrating documentary portrait) is an artist and film-maker who has spent half a century trying to keep himself out of public view.

The new feature about him by Robert B. Weide (of Curb Your Enthusiasm fame) is a shortened version of a much longer documentary made for the American Masters series on PBS. It does a very fair job of prising its subject into the light. Allen, who clearly trusts and respects Weide, talks about the discipline and working methods that have enabled him to rack up an astonishing 43 films in 43 years. He introduces us to the battered old Olympia typewriter on which he bangs out his scripts. We're taken on a magical mystery tour through the haunts of Allen's childhood and we see archive footage of his formidable mother, Nettie Konigsberg. (She frets that if she hadn't been so harsh with him when he was a kid, he might have been a warmer personality.)

What becomes very apparent is Allen's ruthlessness, his lack of sentimentality and his extraordinary ability to "compartmentalise". At the time his custody battle with Mia Farrow was raging and he was being hounded by the media, Allen was somehow able to keep on working as normal on his new feature, Bullets Over Broadway.

Farrow declined to appear in the film but did sign a release so that Weide could use footage from her films with Allen, which many see as among his best. Other collaborators and admirers of Allen queued up to be interviewed by Weide, among them Martin Scorsese, Allen's sister Letty Aronson and his old producers Jack Rollins and Charles H. Joffe, who look as if they've just stumbled out of a Damon Runyon story.

Weide traces Allen's emergence as a teenage gag writer; his first teetering steps as a stand-up comedian and his emergence as a film-maker. (He was so furious with the way his screenplay for What's New Pussycat was mangled that he was determined to retain artistic control over future projects.)

Weide underlines how frustrated Allen remains by his failure (in his own eyes) to make a masterpiece worthy of comparison with the work of the great auteurs he so admires.

However, just as in the Harry Belafonte documentary, the film-maker is fearful of intruding too far into his subject's private life. The telling throwaway details about Allen that Weide has given in interviews (for example, his claustrophobia, his chronic shyness and his phobia about touching toilet seats) don't make it into the documentary.

Concentrating on the work is Weide's excuse for not delving in unseemly fashion into his private life. The paradox, of course, is that this informs the work. It's not just tittle-tattle and gossip. Such material helps us understand the motivations of the artist.

Weide's documentary is an excellent guide to Allen's work, just as Rostock's doc does a thorough job in explaining Harry Belafonte's political preoccupations. However, in both films, the personalities of the subjects remain stubbornly out of reach.