The opening scene of Léa Mysius debut feature “Ava” is one of those artfully casual, life-filled tableaux that you could watch forever: On a crowded French beach, in one crammed frame laid out like a colorful 1970s postcard, a dozen little scenarios play out among the sunbathing, splashing, wading, idling holiday-makers. Each beach towel marks a tiny feudal territory. Overweight couples rub suntan lotion into generous flesh. Cackling kids squeal in the sand. These are not the leggy models and neat hotel parasols of the Cote d’Azur; this is a seashore where real people go, a beach for local families. Even so, despite all the busyness, the camera spies a black dog, apparently belonging to no one, cantering through the crowd and alighting on a young girl dozing in the sun. She wakes with a start and stares at the beast with unusual eyes.

It’s the blackness of the dog you notice, so black it’s almost featureless. It’s negative space — a dog-shaped hole in the world. And that’s how it must appear to the girl, Ava (Noée Abita), an unsmiling 13-year old who will shortly discover that her retinitis pigmentosa has advanced more quickly than expected and she will soon go blind. Mysius’ startlingly assured, exquisitely shot “Ava” is a film that doesn’t simply explore the textural possibilities of 35mm film for the hell of it, it makes thematic use of them, to stunning, evocative effect. DP Paul Guilhaume (who is also the co-screenwriter; an unusual pairing of functions that makes sense given the visual nature of the storytelling in a story about vision) creates images of a peculiar richness in which the colors are saturated but the lens seems progressively more stopped-down so that even the brightest sunlight can feel portentous. “She’s blonde and sunny, and I’m dark and invisible” says Ava, self-pityingly comparing herself to her fair-haired love rival. But Ava’s darkness is anything but invisible; it has a glowering luminosity in a film that shines darkly.

Ava is a truculent, troublesome presence, and her emotional weathervane of a mother, Maud (Laure Calamy) who has a baby daughter and no sign of a partner, tries her best to dote on her and be her friend. But there’s a clear lack of understanding between them, and though tearfully promising to make this “the best summer ever” for Ava, Maud is soon engaged in a love affair with a handsome younger man (Daouda Diakhate). When he gently asks Ava why she has no boyfriend, she replies evenly, “Because I’m mean.” And though that seems fairly true (refreshingly, neither Mysius nor the excellent Abita are at all invested in making Ava likable), she soon has a couple of romantic prospects on the horizon.

Mathias (Baptiste Archimbaud) is the clean-cut, uncomplicated son of her sand-surfing instructor: he’s the right boy for the girl she’s supposed to be. But Ava is not who she’s supposed to be and her failing sight seems to make her ever less so. So there is also a wrong boy. Juan (Juan Cano) is the owner of the dog, which Ava steals on a whim, renames “Lupo” and then returns after she discovers Juan injured from a fight and hiding out on a deserted stretch of beach. From here, the film assumes a borderline “Badlands” status. You can practically taste the wild salt tang of the sea air and sense the youthful abandon in their lovers-on-the-run romance — a place that’s is million miles away from the middle-class normalcy she’s irreversibly left behind.

The film is confidently enigmatic — hiding its secrets behind cloudy eyes. Ava’s loss of sight perhaps mirrors her loss of innocence and coming of age. Lupo functions both as a plot device and, like Churchill’s famous “black dog of depression,” as a metaphor for the wild, engulfing darkness that’s closing in on Ava. And the shooting format, if you would, makes a broader formal point: The imminent obsolescence of 35mm film provides a kind of metatextual parallel with Ava’s failing sight. Right up to the slightly unsatisfying ending, “Ava” is both a complex character portrait and a heartsore farewell to the ephemeral images that will be among the last she sees. But the movie is also, in a way, a tribute to shooting on film, which already feels like an act of radical nostalgia. These are the last wild flickers before darkness.