Hollywood maverick Steven Soderberg’s Unsane is a dizzying, claustrophobic ride, following Sawyer Valentini (Claire Foy) as she becomes admitted to an insane asylum against her will. The movie isn’t particularly about whether she is sane or not, but rather the gradual and unseemly encroachment of her stalker, David Strine (Joshua Leonard). And really, the movie isn’t about the stalker either—it’s about the fact that it was shot entirely on an iPhone.

The woman behind me in the cinema, before the screening, says, “I’m only watching this so I can be part of the conversation”. I’m only watching this because I’m interested as to what an iPhone video looks like on the big screen. Soderberg’s decision seems reminiscent of those gigantic, alluring billboards up near the airport, displaying astounding, meticulous photographs of cityscapes, of people’s faces, with that proud white font—“shot on the iPhone”—destroying all preconceptions of camera-phone capabilities. Is this what technology has become? Is the iPhone singlehandedly destroying the market for DSLRs? And now, with Tangerine and then Unsane, is it destroying the market for professional video-cameras? Can everyone make a feature film worthy of the big screen? The answer is: yes, no, no, and yes, but it’ll look shit. (“It looks like velvet”, Soderberg says, starry-eyed. It doesn’t look like velvet—35mm film looks like velvet. This looks like thin, crummy one-ply toilet paper). The quality of the iPhone video is not fantastic, and it’s made blaringly obvious on a cinema screen—at no point do you forget that the movie is not being filmed with a professional camera.

The technical flaws of the film—the odd focus, the moving grains, the awkward starkness— are instrumental to its success in its tonal inimitability. It carries a raw, untempered texture that is distinctly recognisable, drawing to mind the quality of security camera tapes, of documentaries, or home videos, and as such becomes connected to reality simply through the lens through which it is shot. It has that almost ubiquitous deep focus that strips the frame of any emphasis, and limits artistic indulgence. Soderberg does find ways to depart from student film mediocrity though, through colour grading, professional sound recording, soundtrack, superimpositions, fishbowl distortions, strange low angles, gliding shots, and so on, adding flourishes on what can be best described as footage.

The budget for this film is 1.5 million dollars, extremely low by industry standards, but high for the layman. And so the point here is not that everyone can make a movie (or everyone with a smartphone can make a movie), it’s that camera matters. Whether it’s digital, film, or of the telephonic variety, the camera has the power to fundamentally transform a film. Here the inelegant grit of the lifeless, prosaic smartphone grain elevates the cheap psycho-thriller narrative into a unique, uncomfortable, and inexpungible experience.

Unsane is in cinemas 25 April.