■ Camera fixed on star Claire Foy, this photo shows Ocean’s Eleven director Steven Soderbergh at work on his latest film, a thriller shot entirely on an iPhone. He discusses the pros and cons…

HE’S not the first film-maker to shoot a movie on an iPhone but he’s certainly the most famous. Steven Soderbergh, the Oscar-winning director of Traffic, Magic Mike and the Ocean’s films, is back with electrifying thriller Unsane, starring The Crown’s Claire Foy as a stalker victim who unwittingly signs up for a 24-hour mental health hospital evaluation.

Soderbergh shot the entire feature-length film on an iPhone 7 Plus and, unencumbered by the usual equipment and trucks required by cameras, found it a liberating experience.

‘It was as close as having the camera be a pen as you can get,’ he says. ‘Or being a painter and having an idea and just walking to the wall and starting to paint.’

Soderbergh had been impressed by Sean Baker’s 2015 film Tangerine, which was shot using three iPhone 5s.

‘That let people know where this technology was heading,’ he says.

The iPhone 7 Plus boasts a dual-lens system for 2X optical zoom and improved low-light capture, and the brand’s best ever 4K digital camera. ‘When you show that on a big screen it looks great because of the resolution,’ says Soderbergh.

For those who want to try this at home, the FiLMiC Pro app (iOS £14.99, Android £11.49) allows for extra levels of control when it comes to exposure, shutter speed, colour and focus. A remote option allows you to control it from a second phone for tricky angled shots where you can’t see the lens.

The entire film was written, shot, cut and even scored with Apple technology (and, no, Soderbergh isn’t getting any kickbacks from the tech giant). Three iPhones were rotated throughout the shoot.

‘We had no technical issues at all,’ he says. ‘I never lost anything. I never had dropouts. I never had a battery problem.’

Soderbergh used three lenses — 18mm (the one he deferred to), 60mm and a fisheye lens for those loopy images as Foy’s character begins to become unhinged — designed by the photographic company Moment.

Soderbergh admits there were some issues that had to be tackled. ‘The [iPhone’s] selective focus issue is something I just had to work my way around. The sensor’s very small, you have incredible depth-of-field and have to orient yourself towards things that take advantage of the fact everything’s in focus, for the most part.’

Then there was the phone itself, being so small and light compared to a normal digital camera.

In the opening scene, when Foy’s character Sawyer is answering the phone at her desk, when she placed the receiver down, the ‘camera’ shook. ‘It was below frame,’ says Soderbergh, ‘and I said, “You’re literally going to have to put the phone down like this [very gently].” That means there’s certain kind of shots you can’t do.’

A case in point was a travelling shot of Foy in a car with the phone-camera mounted on the back of the vehicle.

‘The first time I saw it with the phone, it was unusable — it was shaking so much,’ says Soderbergh. ‘I didn’t want to take the time to add 10lbs to the little cage that was holding the phone. I just took the DSLR [camera] that we had on set and put it on the mount and shot it with that.’

Soderbergh is using the iPhone to shoot his next movie, NBA drama High Flying Bird, starring André Holland. Will everybody be doing it soon?

‘It’s not appropriate for everything,’ he says. ‘Some people can’t wrap their mind around doing a $20 million film and shooting it on a phone that costs $700.’

■ Unsane is in cinemas from March 23

More camera less crew

PART of the joy of using a smartphone to shoot your film is that you can travel light — extremely light.

‘All of the camera package fit into a backpack,’ says Steven Soderbergh of shooting Unsane entirely on an iPhone 7 Plus.

And not just kit, crew could be cut back too. ‘I had one grip [they manipulate the camera on cranes or other equipment], one electrician and the camera department was just the assistant cameraman.’

Yet just because you’re going light doesn’t mean you have to limit your shooting style. While old-fashioned cameras are often mounted on dolly tracks, allowing them to smoothly follow action, Soderbergh came up with a more cunning ruse: a DJI stabiliser, used to hold the phone, and a good old-fashioned wheelchair, allowing the camera operator to glide along capturing the moments.

Now that’s thinking on (or off) your feet.

Essential accessories for smartphone film-makers

Gorillapod Mobile Rig

Joby’s GorillaPod tripod has an ultra-flexible design that comes with arms for up to three add-on accessories to complement your smartphone set-up. £89.95, apple.com/uk

DJI Osmo Mobile 2

DJI’s next-gen smartphone stabiliser gimbal makes for smooth video and stills with as little camera shake as possible, even when in motion. £129, jessops.com

Olloclip Filmer’s Kit

The king of add-on lenses has this box of tricks filled with five types, including fisheye and super-wide, alongside a nifty articulating handheld phone grip for more stabilised shooting. £219.95, apple.com/uk

Mega Tiny Anti-Gravity Case

This clever anti-gravity case allows you to fix your phone to mirrors, whiteboards, kitchen cabinets, tiles and most flat surfaces, almost as if by magic. Your arms will thank you. From £17.99, amazon.co.uk