A FEW YEARS AGO, eating popcorn at the opera would have been considered the ultimate in gauche. The sound of crunching kernels would have elicited a chorus of sneers from the rows of tiaras and tuxedos. Sourpussed dowagers, lowering their opera glasses, would stare icily at the boor who slipped by the ushers.

But a recent a spate of opera films are making it possible to munch away on popcorn, Junior Mints or anything else while basking in some of the world's sophisticated music. The film adaptations of Mozart's "The Magic Flute", Puccini's "La Traviata" and now Bizet's "Carmen" return opera to its intended audience, the general public, more successfully than any low-budget opera company ever could.

Yet, as Francesco Rosi's "Bizet's Carmen" wonderfully demonstrates, adaptation is not quite the word to use when discussing opera films. Unlike in movie versions of novels and even plays, where directors often take great liberty in editing and rearranging the material, Rosi remains remarkably faithful to the staged production.

THE JOINING OF two art forms is, nevertheless, tricky, because the task of fitting an opera into the framework of a film risks compromising or distorting the raw material itself. Fortunately, it's not so in Carmen. Here, the medium of film serves only to complement the staged production.

Where before we had only sets, now Rosi gives us the dramatic Spanish countryside; instead of a skeletal chorus we have a carousing corps of dancing peasant women. The film adds to the opera's sensuality without detracting from its heart. Save for the Flower Song scene, Rosi carefully avoids relying on facial close-ups and gestures which--though a necessary cinemagraphic technique--would have been absent from a stage version. In Carmen, the music's the thing.

And what music. Certainly not the most delicate of operas, Carmen roars on for two and a half hours in a mixture of traditional belle canto and lilting Spanish melodies. For those uninitiated into opera, Carmen is a perfect way to begin. Because it relies on a few key songs, most notably the famous "Habanera" and "Toreador's Song," the film is easy to follow. Perhaps the only drawback here is that, despite the Orchestre Nationale de France's energetic performance under conductor Lorin Maazel, Dolby remains a poor substitute for the crispness of the opera hall.

"Love's a gypsy child," sings Carmen (Julia Mignes-Johnson), "he finds that following the rules is hard/If you don't love me, I love you/And if I love you, then be on your guard." And with these lines from "Habaneras" the stage is set for the fateful love triangle involving gypsy Carmen, Don Jose (Placido Domingo) and the bullfighter Escamillio (Ruggero Raimondi).

THE ACTION BEGINS in 19th century Seville, where Don Jose meets Carmen, a worker in a cigarette factory. Captivated by her earthy sensousness, Don Jose soon deserts the army to go off and live with Carmen in the mountains with a band of fellow gypsy smugglers.

But like the love she sings of earlier, Carmen is a "rebellious bird/That you can never hope to tame" and before long the lovers quarrel. Suddenly, Escamillio, the matador, appears. Escamillio also has fallen madly in love with the enchantress since meeting her before her flight from Seville. He challenges Don Jose to yield Carmen but Don Jose, determined to hold onto the last strands of his dignity, refuses. Only when faced with the news that his mother is dying back home does Don Jose leave the smugglers. But all the while he vows to come back and reclaim Carmen.

When he finally makes his way back to Seville a few months later, Don Jose finds Carmen, won over by the matador, at the bullring watching Escamillio in action. Beckoning to meet him outside, he begs her to be his love again. The closing aria is one of alternating pleading and denial in which Carmen, refusing to become a caged bird, falls victim to Don Jose's blind rage.

The individual performances are nearly flawless. Mignes-Johnson combines a mesmerizing screen presence with a powerful voice. Both tenor Domingo and baritone Raimondi are also on the mark, although their acting is stiffer than Mignes-Johnson's. One only hopes that the soundtrack has been doctored as little as possible, because that would add a technological taint to what appears to be a wholly natural and soaring film.