Da Camera Singers Director Sheila Heffernon wasn’t hiding her exasperation in addressing her tenor section after a recent rehearsal preparing the group for a 50th anniversary concert coming up May 11 and 12.

“Watch me! Watch me! Watch me!” she wrote on measure 12 of William Byrd’s “Agnus Dei” from “Mass for Four Voices.” After the session, she sent to all 50 choir members a fierce exhortation in a single-spaced, six-page compilation of sometimes excruciatingly detailed comments.

“Be ready to feel this in one (beat) and make it dance!” she wrote about Monteverdi’s “Cantate Domino.”

Other comments drilled down to musical minutiae: “Feel the hemiola in measures 11 and 12, where you effectively move from two measures of 3 beats to three measures of 2 beats.”

“For the Love of Singing! A Patchwork Quilt of our Favorite Pieces” is the title of the anniversary concert. Her singers know that Heffernon is all heart, but at rehearsals sometimes her great love can feel like tough love.

Da Camera Singers has become a fixture and integral part of the fabric of music making in the Valley since mid-’70s, when several small informal groups of madrigal singers in Amherst, many of them made up of academics and early music devotees, decided to combine forces and hired Hampshire College conductor and composer Ann Kearns to lead them on.

Heffernon — who retired two years ago after 42 years of choral work conducting and teaching at Northfield Mount Hermon School where she was credited with reviving the school’s fame as “the singing school” and departed with the music hall named in her honor —started in 2006 as the group’s fourth regular conductor after John Maggs and Gregory Hayes.

Her well stocked musical tool kit and teaching chops, applied with a good dose of humor she honed from years of working with sometimes unruly adolescents, are these days well exercised in rehearsals with the mostly gray hairs in Da Camera. Working through pieces sometimes note by note, phrase by phrase, with constant attention to pitch and to diction, often feels more like hard work than recreation on a Wednesday night, but her singers embrace the workout, like going to a gym for the voice.

Rehearsal warm-ups tend to go long in order to prime the lungs and flex the pipes. Group dynamics is the cure she applies to achieve a pure and unified and nuanced sound from “mature” voices that individually tend to tremble with vibrato and might get easily winded.

“Make sure your voice is blending! The whole group should be breathing together!”

The fruits of this assiduous joinery are a unified sound both clearer and smoother and sharper than the sum of its parts, the enhancement of the joy of singers who through singing feel rejuvenated, and the gratification of being able to tackle a challenging repertoire that includes pieces of subtle emotion and poetic expression.

“We are a serious choir doing serious music,” says longtime President Frank Couvares, professor of history at Amherst College. “Sheila knows how to work us and get a good tone quality out of us.”

Da Camera’s half-century-old archive bulges with more than 1,000 compositions ranging from that early early music right through every major musical period to the present, which is beautifully represented in a significant body of new work the group has commissioned from area composers.

“We have brought new music to life,” says Couvares. “We’ve brought to life music nobody ever heard. Isn’t that a wonderful thing?”

High points for the chorus looking back include a 40th anniversary reprise of commissions, the Monteverdi Vespers, Richard Einhorn’s haunting music for Carl Theodor Dreyer 1929 silent film “The Passion of Joan of Arc,” that was performed alongside a screening of the movie at the Academy of Music in Northampton, and more recently, the epic oratorio by Craig Hella Johnson, “Considering Matthew Shepard,” about a gay teenager brutally murdered in Wyoming, with lyrics by Valley poet Leslea Newman.

This latter undertaking represents a trend towards organizing concerts around themes of contemporary social significance that has been one of Heffernon’s hallmarks.

She remembers longtime choir member Jefferson Hunter urging her at one point, “I think we need to use our voices to speak out, to sing out, about things that make us angry in this world.” She could not have agreed more.

For Heffernon, Da Camera hit a peak with Bach’s “Mass in B minor,” sung 10 years ago, the fruit of a year of rehearsals in which “we pulled apart the phrasing into tiny bits, and the singers put heart and soul into every note until the music became part of the fabric of their very souls.”

Selections of the group’s own favorites from the baroque, renaissance, romantic, and modern periods, as well as new work by Alice Parker, Eric Sawyer, Ronald Perera, Lewis Spratlan, and Craig Sanford, are featured on a packed program of fairly short samples for the anniversary concert.

The program is organized in sections on a theme of “gratitude” — for music, for humble beginnings, for the sacred, for the natural world, for love and for each other.

In the section titled “In Gratitude for Former Members” is tucked a brief, hushed number, “A Shade upon the Mind” for tenors and basses. Taken from “Heavenly Hurt,” Alice Parker’s choral setting of poems by Emily Dickinson, it will be sung with an emotion the years only ripen.

A little sentiment can sweeten the singing, Heffernon would be the first to admit, but on the other hand the way an old choir stays strong and young is by consciously and conscientiously embracing the new.

Even as Da Camera looks back on its long, rich evolution, it is continuing its commitment as a “commissioning choir” and to new music and in support of young talent coming up. Next year they will hold a competition for high school composers, and they will include the winning compositions in a concert a year from this spring to be entitled “Celebrating Teenagers — Old and New” that will include those famously young geniuses, Mozart and Schumann.

“For the Love of Singing” will be performed Saturday, May 11, at 7:30 p.m. at the First Congregational Church in Amherst and Sunday, and Sunday, May 12, at 3 p.m. at St. James and St. Andrew Episcopal Church in Greenfield. The concert is free, with donations gladly accepted.

Judson Brown is a retired journalist and social service worker who is continuing to write, photograph, volunteer and sing as a member of Da Camera Singers.