Nicholas Nestorak (left) as Tobias Ragg and members of the ensemble in The Glimmerglass Festival's production of Stephen Sondheim's "Sweeney Todd." (Karli Cadel/The Glimmerglass Festival)

What’s the difference between an opera and a musical? Theories abound. Some say opera is through-sung while musicals include spoken dialogue (although there are many operas with spoken dialogue; think “Carmen”). Some say opera is unamplified, while musicals are mic’d (except that there are many operas written for amplification, and many musicals that weren’t).

But ask Stephen Sondheim, as Jamie Bernstein did in a public discussion on Saturday afternoon at the Glimmerglass Festival, and you’ll hear that it has to do largely, simply, with the expectations of the audience.

Sondheim should know — not least because his “Sweeney Todd” seems to have jumped the much-debated divide and been accepted into the opera canon. Indeed, Glimmerglass’s production of it by Christopher Alden, seen Saturday night, illustrates another difference between opera and musicals: Musicals are generally presented in a somewhat realistic setting, while operas are often produced in ways that have little to do with the creators’ original vision.

By this token, Alden’s “Sweeney Todd,” set in a drab 1960s-vintage community hall that is then deconstructed into early 20th-century music hall components, played to the expectations of an opera audience, which has at least seen this kind of thing before. Not that they necessarily like it.

Sondheim didn’t set out to make “Sweeney Todd” operatic. If anything, he said, “it’s a movie, with movie music” (including leitmotifs, a staple of film scores). As for adaptation: One secret to success in Sondheim’s book, be it Puccini operas or “Sweeney Todd,” is slender source material. “Puccini took kitsch plays,” he said, “and ennobled them.”

Those interested in direct comparison of the two forms will have a chance in New York in 2017, when Sondheim’s next musical, written with the playwright David Ives and based on two movies by Luis Buñuel (“The Discreet Charm of the Bourgeoisie” and “The Exterminating Angel”) will open at the Public Theater in the same year that Thomas Adès’s opera “The Exterminating Angel,” which just had its premiere in Salzburg (to enthusiastic reviews), is scheduled to come to the Metropolitan Opera.

But this year’s Glimmerglass Festival offered its own direct comparison between opera and theater with its two American works. “Sweeney Todd,” call it opera or no, filled this year’s “musical” slot, in Glimmerglass’s new tradition of presenting a musical with a full orchestra and without amplification. The lack of amplification spotlights another opera/musical divide: The songs in musicals tend to lie mainly in the lower part of a singer’s voice. If you don’t have good “belt” technique — not generally encouraged for opera singers — it is hard to make an impact without a mic, especially over a full orchestra, as Patricia Schuman’s Beggar Woman demonstrated, although conductor John DeMain toned down the volume as best he could.

Brian Mulligan as John Proctor and Jamie Barton as Elizabeth Proctor in Robert Ward's "The Crucible." (Karli Cadel/The Glimmerglass Festival)

Then there was the 1961 opera “The Crucible,” which the composer Robert Ward based faithfully on Arthur Miller’s play. “The Crucible” supports Sondheim’s theory about shallow source material by illustrating its opposite: The original play is so powerful that Ward’s capable, earnestly pretty music had little to add and only diminished the drama with melodramatic flourishes (melodramatically conducted by Nicole Paiement in the pit) and by abridging the text and characters.

Glimmerglass cast it powerfully, with Brian Mulligan and Jamie Barton in redoubtable vocal form as the Proctors; Ariana Wehr as a searing Abigail Williams; and David Pittsinger as the initially misguided but fundamentally decent Reverend Hale (a character description that’s squarely in Pittsinger’s wheelhouse). Jay Hunter Morris, this year’s artist-in-residence, made some awful sounds as the awful Judge Danforth. But none of this really elevated the piece. If there was any movie music at Glimmerglass this summer, this was it.

It wasn’t “Sweeney Todd,” whatever Sondheim says. It takes something operatic to hold its own and still have dramatic effect through such a distracting staging. As Alden deconstructed his way through the epochs of the play’s various source materials, the singers had to do most of the heavy lifting because the sets and costumes didn’t help much. Yet the piece still caused chills — such as the moment when Greer Grimsley’s Todd, though prone to overacting, realized with a cry that he had killed his own wife.

Glimmerglass’s young artists had plenty of chances to shine in both pieces. Emily Pogorelc offered a Johanna with striking vocal color and depth and drama, and Molly Jane Hill, the cover singer for Mrs. Lovett, was assured and funny in a tough role. Harry Greenleaf showed a penetrating lyric tenor as Anthony Hope.

But with operatic voices in a supposed musical, you’d expect better, richer sound, overall, and a lot of the singers had a terrible time finding the right pitch at the beginning of their numbers. This problem was so pervasive that it had to be something about the staging or acoustics that was tripping them up. Having already committed the heresy of proclaiming “Sweeney Todd” an opera, I will compound my sins by suggesting that, especially when a piece was originally written to be mic’d, some judicious amplification might actually help.

The Glimmerglass Festival continues through Aug. 27, and also includes performances of “La Bohème” and “The Thieving Magpie.” Visit