Ruth Negga, left, and Tessa Thompson play Black women, one of whom “passes” for white, in the 1920s-set “Passing.” (Netflix/The Minnesota Star Tribune)

Ironically, the black-and-white cinematography in "Passing" is there to show us that nothing in the drama is black and white.

Beginning on a blazing hot day in 1920s Harlem and ending on a frigid one, "Passing" is all about shades of gray — which, of course, is also the truth of black-and-white films. They are really many shades of gray.

Rebecca Hall's adaptation of Nella Larsen's novel is about two Black friends who reconnect after several years: Irene (Tessa Thompson) and Clare (Ruth Negga), whom Irene learns has been "passing" as a white woman.

It's complex, provocative material, especially since Clare is married to a man who is not just white but violently racist ("I hate them!" he says of "Negroes," unaware he is married to one). By homing in on a brief period in the friendship of the women, Hall (better known as an actor from "Vicky Cristina Barcelona") creates a work that is likely to make many viewers measure their own experiences against the ones of the characters.

"We're all of us passing for something, aren't we?" Irene asks, shortly after attending a party whose guests, sharp-eyed viewers will note, include a transgender woman. And, of course, not all of our "passing" is as evident as that. Some shy people manage to pretend they're outgoing. Some gay people pass for straight. Some adulterers pose as single, at least until they're caught.

Although this is the first film written and directed by Hall — who identifies as mixed race and some of whose ancestors "passed" for white — it is an astonishingly confident one. Her decision to shoot in black and white made it harder to get the film made but it's a perfect one. That is not only because of the shades-of-gray thing but also because it makes the actors' jobs easier, particularly Negga, whose "whiteness" appears to have been achieved mostly with filters and lighting.

In a similar vein, Hall chose a tight, almost-square frame, which works both as an approximation of the silent films that would have been popular in the '20s and a way to confine the characters in a smaller, more claustrophobic square.

Not surprisingly, Hall also does great work with the actors. Negga's Clare has a little Blanche DuBois in her. She's delicate and jittery, which makes sense since she spends every minute in fear that her husband and others will discover her secret. Thompson's wary Irene feels compassion for her friend but she's also reluctant to get too close to her, perhaps because she worries Clare will be perceived as Black by association and perhaps because she understands why Clare does what she does more than she'd like to admit.

As the women interact, the question stands out because it's barely addressed: Why would Clare do it? It's presumed there are good reasons she places herself in this precarious position. When she can drown out her fears, it's true that aspects of her life are easier than Irene's, even if Irene is obviously happier with her (Black) husband and kids, whom she tries to shield from "the race problem" as much as she can.

Clare, the movie implies, chose to "pass" because it was safer not to be Black in a white supremacist world that didn't recognize her humanity. The last time we see her, in fact, she is lying on a field of snow, surrounded by — smothered by — whiteness.