IN THE MINDS of many film critics and committed moviegoers, Sidney Lumet has been tried, convicted and sentenced. His offence against the entertainment world: wilfully tampering with Peter Shaffer's masterpiece Equus in Lumet's film adaptation of what may be the play of the '70s. Reviewers have pilloried Lumet for abandoning the example of the record-shattering Broadway production, and instead applying his personal imprint to the movie version. But such bad-mouthing has an unfair ring; if the director had retained the heavy-handed horse symbolism of the stage version, the critics' court probably would have found Lumet guilty of emulation when his duty called for an imaginative adaptation. Acknowledging his obligation to present a different slant, Lumet has stripped the story of its dependence on icon-like imagery and lent the narrative a stamp of realism that is quite appropriate to the special demands of cinema. With considerable assistance from the shining performances of Richard Burton--his finest in recent memory--and the promising Peter Firth, Lumet has successfully carried out the delicate operation of transforming Shaffer's impressionistic play into a full-length strip of celluloid.

The opening portions of Equus unveil the two recurring images that will dominate the film's visual dimension: a close-up of the doubt-ridden psychiatrist Martin Dysart (Burton) musing about the complex case of his teenage patient Alan Strang (Firth), and a darkness-clothed scene of a naked Strang standing beside a horse, the object of his near psychotic obsession. Lumet fills his lens with Dysart's ruminating face, punctuating the narrative with the Shakespearean soliloquies of the confused shrink. At times, these infrequent monologues border on the histrionic, as Burton casts off the necessary restraint of a film star and takes on the exaggerated inflections of a stage actor's voice. But on balance, the technique succeeds, enabling Burton to dedge up the personal torment that his patient Strang has triggered within him.

"That boy has known a passion more ferocious than any I have ever known throughout my life, and I envy him for it," Dysart declares to the audience, making an eloquent summary of the dilemma that the psychiatrist perceives as he approaches Strang's case. In the figure of the disturbed boy, Dysart has run up against a patient who matches his own forceful character, yet can identify Dysart's very unique weaknesses with all the insight of one of the psychiatrist's professional colleagues. Strang senses the sexual and emotional impotence of his putative healer, using his instinctive acumen as a weapon to retaliate against Dysart's incursions into his own psyche. Strang's special qualities compound the difficulty of the shrink's task; the adolescent's mercurial nature furnishes a painful counterpoint to his doctor's sterile intellectualism. Strang forces Dysart to tackle his own neuroses--which seem so pallid by comparison--while grappling with the wrenching monomania of the young patient. He is, in a clinical sense, the judge and the judged, by his own will.

The key to understanding the extraordinary success of Shaffer's work (he also is responsible for the film's screenplay) lies in the fundamental issues of existence raised by Dysart's mental meanderings. In his search to uncover the sources of Strang's obsession, Dysart observes that "a child is born into a world of phenomena all equal in their power to enslave." But the psychiatrist can only throw up his hands with an admission of resigned incomprehension: "Why those moments of experience are particularly magnified no one can say." From one perspective, the psychiatrist's reflections seem a recognition of the paradox that plagues his profession; he has chosen to devote his life to understanding the human psyche, something that cannot ever be fully understood. Yet his statements have a broader significance for the audience, challenging each individual to justify his existence. As Dysart quotes the young Strang, paying homage to the portrait of a horse: "Account for me, Equus."

BUT LUMET'S FILM does not limit itself to playing out fascinating mind games. As might be expected from a movie based on a play bursting with totem-like symbols, Equus is studded with the type of rich imagery that will easily come to mind with the mere mention of the film's title. Many viewers of Equus will fasten onto the scene of the nude Strang riding a madly galloping steed, a union of man and beast thrown into relief by backgrounds that alternate between the darkest of nights and a blinding brilliance of light. But the segment that qualifies as Lumet's tour de force lies elsewhere, in the scene showing Strang's brutal rampage through a stable--sparked by his failure to make love to young girl--that eventually leads him to Dysart's couch. After he has brusquely dismissed the girl out of frustration and shame, the unclothed Strang stands fully erect in the loft of the stable, hurling himself into the now familiar rite of horse adulation. Suffused with ambers and golds, the shot evokes a tinted Renaissance fresco, with the rapturous Strang projecting a seeming embodiment of innocence and worship. But then Strang shatters this almost transcendental effect, suddenly jumping down from the loft and beginning an orgy of violence, where he wantonly blinds six horses.

This graphic display of savagery is one of several similar scenes that have appalled viewers of Equus who prefer the tamer stage version of the work. An equally testing juncture shows a kneeling Strang in his room, a makeshift harness with reins attached to his head, beating his right thigh with a stick that passes for a riding crop, as his appalled father looks on. Ultimately, the treatment of these segments may certainly seem gratuitous, but Lumet did not aim at merely shocking his viewer. Rather, he tries to underscore the intensity of his protagonist's monomania.

In view of the challenge he set himself, Lumet deserves pardon for a few tactical mistakes. He has come up with a film sufficiently slick and commercial to avoid the stigma of a pseudo-Bergman exploration of the soul, yet without cheapening the gravity of the questions that arise from the struggle between Dysart and his young patient. Much of the credit for this achievement must of course go to Shaffer's extraordinary script. But such a nod to the playwright in no way lessens the triumph of the man behind the camera.