The only effective aspect of the new science-fiction trifle "Spacehunter: Adventures in the Forbidden Zone," now at area theaters, is its 3-D cinematography, which achieves fairly sophisticated and disarming depth illusions from one setting to the next. Evidently, the decision to shoot in 3-D was made on short notice and Lamont Johnson was hired to replace the original director on equally short notice.

But the general assurance of the pictorial side of the film doesn't go as far as it should, because the script is too undernourished and derelict to sustain even fleeting human or dramatic interest. If a capable director had been engaged in time to supervise a desperately needed script overhaul, the movie might have generated some staying power. In its present half-baked state, the pictorial effects can't compensate for the rickety story material and characterization.

"Spacehunter" looks like a one weekend exploitation movie. It already suffers from the downcast, underpopulated, minor-league aura of a picture that belongs on the bottom half of double bills.

The principal characters in "Spacehunter" are located on a desolate, plague-infested planet called Terra Eleven, but you can't help noticing that it's the company that seems stranded, lost on picturesque badlands locations near Moab, Utah, without nearly enough screenplay to keep them going.

Thanks to a consistently garbled soundtrack, I also found it impossible to sort out the principal characters and their ostensible functions until I resorted to the plot synopsis supplied by the Columbia publicity department. Peter Strauss is the hero, a taciturn loner called Wolff who lands on uninviting Terra Eleven in pursuit of the reward offered for three starlets unfortunately stranded there and left to the loathsome mercies of the diseased, ravaged native populace, or its monstrous remnant. Wolff acquires a bratty companion named Niki (Molly Ringwald, who played the daughter in Paul Mazursky's "Tempest"), the survivor of some ill-fated medical mission of mercy. She offers her services as a tracker and kibitzer and utterly fails to ingratiate herself as an adorable hoydenish pickup. In fact, there's something rather unsavory about the feeble effort to cook up a romance between the impertinent teen-ager and the strong silent hero, who's bound to end up looking like a strong silent cradle-robber, too.

These mismatched desert rats meander around the arroyos in a kind of futuristic jeep, and every so often creepy-crawly predators emerge suddenly and arbitrarily from the shadows to lunge or slobber at them. Consistently inept at entrapment, the ghoulish parties then vanish as inexplicably as they appeared. The screenwriters can't seem to think up other situations, so Wolff and Niki keep almost getting snatched until they finally straggle into the lair of an archfiend called Overdog (Michael Ironside, who was the menace in "Scanners"), half-man and half-machine, who evidently runs things on rundown Terra Eleven and has the three marooned dolls in captivity. It's taken the hero so long to locate the villain that you're surprised the captives haven't been brutalized into mincemeat, but they've somehow been spared unspeakable fates and return safe and sound after Wolff outfoxes Overdog. While there obviously would be a certain logic about Wolff accepting a sexual bonus from the grateful trio, he seems mysteriously resigned to indulging Niki's desire to become his adoring consort.

An actor named Ernie Hudson shows a welcome heroic wit and flair in the role of some kind of maverick lawman who is inserted arbitrarily to share fleeting interludes of facetious, mock-antagonistic, between-us-studs repartee with Strauss. In fact, this minor character seems a more promising model for a science-fiction adventure hero than the official hero; he's closer to George Miller's Mad Max before being ravaged by bitter disillusion--a tough sheriff working the toughest beat in the solar system. Maybe the producers of "Spacehunter" can recoup by building a television series around Hudson as a spacepatrolman. SPACEHUNTER: ADVENTURES IN THE FORBIDDEN ZONE

Directed by Lamont Johnson; screenplay by Edith Rey, David Preston, Dan Goldberg and Len Blum; story by Stewart Harding and Jean LaFleur; director of photography, Frank Tidy, B.S.C.; edited by Scott Conrad, A.C.E.; music by Elmer Bernstein; produced by Don Carmody, John Dunning and Andre Link; executive producer, Ivan Reitman. A Columbia Pictures presentation. THE CAST Wolff . . . . Peter Strauss Niki . . . . Molly Ringwald Washington . . . . Ernie Hudson Chalmers . . . . Andrea Marcovicci Overdog . . . . Michael Ironside