The final scene of Luca Guadagnino’s Call Me By Your Name plays while the credits roll – prompting audiences to object to the house lights being turned on. So how do theatres decide when to flick the switch?

Italian film-maker Luca Guadagnino’s new coming-of-age drama Call Me by Your Name has received ecstatic reviews, but not every cinemagoer has emerged from the movie content. According to reports on social media, branches of Vue and Odeon chose to raise the house lights as the credits rolled – even though they roll over the film’s moving final scene.

The scene may not be especially crucial to the plot, but it contains the last beats of an awards-worthy turn by its young star, Timothée Chalamet. So is flicking the light-switch the senseless act of a multiplex with no appreciation for the subtlety of arthouse cinema? Actually, no, says Dave Norris, a former chief projectionist at the Empire cinema in Leicester Square, London.

For several years, he explains, it has been a legal health and safety requirement that cinemas put their lights up whenever audience members are likely to be moving around; that goes for the credits at the end of a film, as well as the trailers beforehand. Now that most cinema projections are automated, there’s rarely a human in the booth exercising personal discretion.

Movies arrive at cinemas not on a reel, but on a hard drive. “The drive has a label on it with the film’s title, the length of the movie, what screen ratio it should be shown in and so on. And it also has the credit offset time,” says Norris. “The cinema puts a cue in their automation system, which will bring up the lights at precisely that moment in the screening.”

Most cinemas have a “subtle” lighting setting for the credits, which ought to satisfy both the safety rules and cinemagoers. But, Norris adds: “No two lighting levels are going to be the same from cinema to cinema.” In an age when more and more films have mid-credit and post-credit sequences, “when to bring up the lights is a grey area”.

Marvel movies famously feature multiple post-credits sequences, which often set up the next film in the superhero franchise. Modern comedies frequently include blooper reels after the film itself is over. The trend extends to arthouse: in his acclaimed 2005 film Caché (“Hidden”), director Michael Haneke revealed a crucial twist in a wordless scene during the final credits.

Corinna Antrobus, communications manager at Picturehouse, says most of its cinemas have projectionists, “who can apply a judgment as to when to put up the lights and by how much”. In the case of Call Me by Your Name, they would “aim to keep the lights low or off until … the scene has completed.”

Vue, meanwhile, confirms its lighting has always been “automatically sequenced to come on to a half-light as the credits start, for customers wishing to leave the screen immediately”. If customers complain, the chain says, it checks lighting levels for the screenings in question, adding: “We do not have different lighting policies for different genres of films.”