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Now you see it, now you don't: what optical illusions tell us about our brains

Illusions can offer insights into how the visual system processes images.

Maurits Escher: where do the staircases lead?

The human brain is a network of about 20 billion neurons – nerve cells – linked by several trillion connections. Not to mention glial cells, which scientists used to think were inactive scaffolding, but increasingly view as an essential part of how the brain works. Our brains give us movement, language, senses, memories, consciousness and personality. We know a lot more about the brain than we used to, but it still seems far too complicated for human understanding.

Fortunately, the brain contains many small networks of neurons that carry out some specific function: vision, hearing, movement. It makes sense to tackle these simple modules first. Moreover, we have good mathematical models of nerve cell behaviour. In 1952, Alan Hodgkin and Andrew Huxley wrote down the “Hodgkin-Huxley equations” for the transmission of a nerve impulse, which won them the 1963 Nobel Prize in Medicine. We also have effective techniques for understanding small networks’ components and how they are linked.

Many of these simple networks occur in the visual system. We used to think that the eye was like a camera, taking a “snapshot” of the outside world that was stored in the brain like a photo stuck in an album. It uses a lens to focus an image on to the retina at the back of the eye, which functions a bit like a roll of film – or, in today’s digital cameras, a charge-coupled device, storing an image pixel by pixel. But we now know that when the retina sends information to the brain’s visual cortex, the similarity to a camera ends.

Although we get a strong impression that what we are seeing is “out there” in front of us, what determines that perception resides inside our own heads. The brain decomposes images into simple pieces, works out what they are, “labels” them with that information, and reassembles them. When we see three sheep and two pigs in a field, we “know” which bits are sheep, which are pigs, and how many of each there are. If you try to program a computer to do that, you quickly realise how tricky the process is. Only very recently have computers been able to distinguish between faces, let alone sheep and pigs.

Probing the brain’s detailed activity is difficult. Rapid progress is being made, but it still takes a huge effort to get reliable information. But when science cannot observe something directly, it infers it, working indirectly. An effective way to infer how something functions is to see what it does when it goes wrong. It may be hard to understand a bridge while it stays up, but you can learn a lot about strength of materials when it collapses.

The visual system can “go wrong” in several interesting ways. Hallucinogenic drugs can change how neurons behave, producing dramatic images such as spinning spirals, which originate not in the eye, but in the brain. Some images even cause the brain to misinterpret what it’s seeing without outside help. We call them optical illusions.

One of the earliest was discovered in Renaissance Italy in the 16th century. Giambattista della Porta was the middle of three surviving sons of a wealthy merchant nobleman who became secretary to the Holy Roman emperor Charles V. The father was an intellectual, and Giambattista grew up in a house in Naples that hosted innumerable mathematicians, scientists, poets and musicians. He became an outstanding polymath, with publications on secret codes (including writing on the inside of eggshells), physiology, botany, agriculture, engineering, and much else. He wrote more than 20 plays.

Della Porta was particularly interested in the science of light. He made definitive improvements to the camera obscura, a device that projects an image of the outside world into a darkened room; he claimed to have invented the telescope before Galileo, and very likely did. His De refractione optices of 1593 contained the first report of a curious optical effect. He arranged two books so that one was visible to one eye only and the other to the other eye. Instead of seeing a combination of the two images, he perceived them alternately. He discovered that he could select either image at will by consciously switching his attention. This phenomenon is known today as binocular rivalry.

Two other distinct but related effects are impossible figures and visual illusions. In rivalry, each image appears unambiguous, but the eyes are shown conflicting images. In the other two phenomena, both eyes see the same image, but in one case it doesn’t make sense, and in the other it makes sense but is ambiguous.

Impossible figures at first sight seem to be entirely normal, but depict things that cannot exist – such as Roger Shepard’s 1990 drawing of an elephant in which everything above the knees makes sense, and everything below the knees makes sense, but the two regions do not fit together correctly. The Dutch artist Maurits Escher made frequent use of this kind of visual quirk.

In 1832, the Swiss crystallographer Louis Necker invented his “Necker cube” illusion, a skeletal cube that seems to switch its orientation repeatedly. An 1892 issue of the humorous German magazine Fliegende Blätter contains a picture with the caption “Which animals are most like each other?” and the answer “Rabbit and duck”. In a 1915 issue of the American magazine Puck, the cartoonist Ely William Hill published “My wife and my mother-in-law”, based on an 1888 German postcard. The image can be seen either as a young lady looking back over her shoulder, or as an elderly woman facing forwards. Several of Salvador Dalí’s paintings include illusions; especially Slave Market With the Apparition of the Invisible Bust of Voltaire, where a number of figures and everyday objects, carefully arranged, combine to give the impression of the French writer’s face.

Illusions offer insights into how the visual system processes images. The first few stages are fairly well understood. The top layer in the visual cortex detects edges of objects and the direction in which they are pointing. This information is passed to lower layers, which detect places where the direction suddenly changes, such as corners. Eventually some region in the cortex detects that you are looking at a human face and that it belongs to Aunt Matilda. Other parts of the brain are alerted, and you belatedly remember that tomorrow is her birthday and hurry off to buy a present.

These things don’t happen by magic. They have a very definite rationale, and that’s where the mathematics comes in. The top layer of the visual cortex contains innumerable tiny stacks of nerve cells. Each stack is like a pile of pancakes, and each pancake is a network of neurons that is sensitive to edges that point in one specific direction: one o’clock, two o’clock and so on.

For simplicity, call this network a cell; it does no harm to think of it as a single neuron. Roughly speaking, the cell at the top of the stack senses edges at the one o’clock position, the next one down corresponds to the two o’clock angle, and so on. If one cell receives a suitable input signal, it “fires”, telling all the other cells in its stack: “I’ve seen a boundary in the five o’clock direction.” However, another cell in the same stack might disagree, claiming the direction is at seven o’clock. How to resolve this conflict?

Neurons are linked by two kinds of connection, excitatory and inhibitory. If a neuron activates an excitatory connection, those at the other end of it are more likely to fire themselves. An inhibitory connection makes them less likely to fire. The cortex uses inhibitory connections to reach a definite decision. When a cell fires, it sends inhibitory signals to all of the other cells in its stack. These signals compete for attention. If the five o’clock signal is stronger than the seven o’clock one, for instance, the seven o’clock one gets shut down. The cells in effect “vote” on which direction they are detecting and the winner takes all.

Many neuroscientists think that something very similar is going on in visual illusions and rivalry. Think of the duck and rabbit with two possible interpretations. Hugh R Wilson, a neuroscientist at the Centre for Vision Research at York University, Toronto, proposed the simplest model, one stack with just two cells. Rodica Curtu, a mathematician at the University of Iowa, John Rinzel, a biomathematician then at the National Institutes of Health, and several other scientists have analysed this model in more detail. The basic idea is that one cell fires if the picture looks like a duck, the other if it resembles a rabbit. Because of the inhibitory connections, the winner should take all. Except that, in this illusion, it doesn’t quite work, because the two choices are equally plausible. That’s what makes it an illusion. So both cells want to fire. But they can’t, because of those inhibitory connections. Yet neither can they both remain quiescent, because the incoming signals encourage them to fire.

One possibility is that random signals coming from elsewhere in the brain might introduce a bias of perception, so that one cell still wins. However, the mathematical model predicts that, even without such bias, the signals in both cells should oscillate from active to inactive and back again, each becoming active when the other is not. It’s as if the network is dithering: the two cells take turns to fire and the network perceives the image as a duck, then as a rabbit, and keeps switching from one to the other. Which is what happens in reality.

Generalising from this observation, Wilson proposed a similar type of network that can model decision-making in the brain – which political party to support, for instance. But now the network consists of several stacks. Maybe one stack represents immigration policy, another unemployment, a third financial regulation, and so on. Each stack consists of cells that “recognise” a distinct policy feature. So the financial regulation stack has cells that recognise state regulation by law, self-regulation by the industry, or free-market economics.

The overall political stance of any given political party is a choice of one cell from each stack – one policy decision on each issue. Each prospective voter has his or her preferences, and these might not match those of any particular party. If these choices are used as inputs to the network, it will identify the party that most closely fits what the voter prefers. That decision can then be passed to other areas of the brain. Some voters may find themselves in a state akin to a visual illusion, vacillating between Labour and Liberal Democrat, or Conservative and Ukip.

This idea is speculative and it is not intended to be a literal description of how we decide whom to vote for. It is a schematic outline of something more complex, involving many regions of the brain. However, it provides a simple and flexible model for decision-making by a neural network, and in particular it shows that simple networks can do the job quite well. Martin Golubitsky of the Mathematical Biosciences Institute at Ohio State University and Casey O Diekman of the University of Michigan wondered whether Wilson’s networks could be used to model more complex examples of rivalry and illusions. Crucially, the resulting models allow specific predictions about experiments that have not yet been performed, making the whole idea scientifically testable.

The first success of this approach helped to explain an experiment that had already been carried out, with puzzling results. When the brain reassembles the separate bits of an image, it is said to “bind” these pieces. Rivalry provides evidence that binding occurs, by making it go wrong. In a rivalry experiment carried out in 2006 by S W Hong and S K Shevell, the subject’s left eye is shown a horizontal grid of grey and pink lines while the right eye sees a vertical grid of grey and green lines. Many subjects perceive an alternation between the images, just as della Porta did with his books. But some see two different images alternating: pink and green vertical lines, and pink and green horizontal lines – images shown to neither eye. This effect is called colour misbinding; it tells us that the reassembly process has matched colour to grid direction incorrectly. It is as if della Porta had ended up seeing another book altogether.

Golubitsky and Diekman studied the simplest Wilson network corresponding to this experiment. It has two stacks: one for colour, one for grid direction. Each stack has two cells. In the “colour” stack one cell detects pink and the other green; in the “orientation” stack one cell detects vertical and the other horizontal. As usual, there are inhibitory connections within each stack to ensure a winner-takes-all decision.

Following Wilson’s general scheme, they also added excitatory connections between cells in distinct stacks, representing the combinations of colour and direction that occur in the two “learned” images – those actually presented to the two eyes. Then they used recent mathematical techniques to list the patterns that arise in such a network. They found two types of oscillatory pattern. One corresponds to alternation between the two learned images. The other corresponds precisely to alternation between the two images seen in colour misbinding.

Colour misbinding is therefore a natural feature of the dynamics of Wilson networks. Although the network is “set up” to detect the two learned images, its structure produces an unexpected side effect: two images that were not learned. The rivalry experiment reveals hints of the brain’s hidden wiring. The same techniques apply to many other experiments, including some that have not yet been performed. They lead to very specific predictions, including more circumstances in which subjects will observe patterns that were not presented to either eye.

Similar models also apply to illusions. However, the excitatory connections cannot be determined by the images shown to the two eyes, because both eyes see the same image. One suggestion is that the connections may be determined by what your visual system already “knows” about real objects.

Take the celebrated moving illusion called “the spinning dancer”. Some observers see the solid silhouette of a dancer spinning anticlockwise, others clockwise. Sometimes, the direction of spin seems to switch suddenly.

We know that the top half of a spinning dancer can spin either clockwise or anticlockwise. Ditto for the bottom half. In principle, if the top half spins one way but the bottom half spins the other way, you would see the same silhouette, as if both were moving together. When people are shown “the spinning dancer”, no one sees the halves moving independently. If the top half spins clockwise, so does the bottom half.

Why do our brains do this? We can model that information using a series of stacks that correspond to different parts of the dancer’s body. The brain’s prior knowledge sets up a set of excitatory connections between all cells that sense clockwise motion, and another set of excitatory connections between all “anticlockwise” cells. We can also add inhibitory connections between the “clockwise” and the “anticlockwise” cells. These connections collectively tell the network that all parts of the object being perceived must spin in the same direction at any instant. Our brains don’t allow for a “half and half” interpretation.

When we analyse this network mathematically, it turns out that the cells switch repeatedly between a state in which all clockwise cells are firing but the anticlockwise ones are quiescent, and a state in which all anticlockwise cells are firing but the clockwise ones are quiescent. The upshot is that we perceive the whole figure of the dancer switching directions. Similar networks provide sensible models for many other illusions, including some in which there are three different inputs.

These models provide a common framework for both rivalry and illusion, and they unify many experiments, explain otherwise puzzling results and make new predictions that can be tested. They also tell us that in principle the brain can carry out some apparently complex tasks using simple networks. (What it does in practice is probably different in detail, but could well follow the same general lines.)

This could help make sense of a real brain, as new experiments improve our ability to observe its “wiring diagram”. It might not be as ambitious as trying to model the whole thing on a computer, but modesty can be a virtue. Since simple networks behave in strange and unexpected ways, what incomprehensible quirks might a complicated network have?

Perhaps Dalí, and Escher, and the spinning dancer can help us find out. 

Ian Stewart is Emeritus Professor of Mathematics and Digital Media Fellow at the University of Warwick

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Can celluloid lovers like Christopher Nolan stop a digital-only future for film?

Despite proponents like the Dunkirk director, physical film is finding it tough in the modern age. 

“Chris Nolan is one of the few producing and directing films right now who could open that film. He is one of the all-time great filmmakers.”

No prizes for guessing which new release Vue CEO Tim Richards is talking about. Aside from its box office success, aside from its filmmaking craft, aside even from its early reception as an Oscar favourite, Dunkirk sees Nolan doing what Nolan does best: he has used his latest film to reopen the debate about celluloid.

Until relatively recently all film was projected from that old, classic medium of the film reel - a spool of celluloid run in front of a projector bulb throwing images on to a screen. It comes mainly in two forms: 35mm (standard theatrical presentations) or 70mm (larger, more detailed presentations most popular in the 60s and 70s). Fans say it provides a “warmer” colour palette, with more depth and saturation than modern digital formats.

But now it’s hard to even see movies on film to make the comparison. After George Lucas, godfather of the Star Wars franchise, shot Star Wars Episode II: Attack of the Clones entirely in digital rather than on physical film, the rollout of digital progressed with clinical efficacy. Within ten years, film was almost wiped out, deemed to be impractical and irrelevant. Modern cinema, it was argued, could be stored in a hard drive.

Christopher Nolan set out to change all that. He championed film as a medium against the industry trend, producing (The Dark Knight, The Dark Knight Rises, Interstellar) in super-detailed, super-sized IMAX 70mm. With Dunkirk, Nolan has taken that further by screening the film in 35mm, 70mm and IMAX 70mm.

Nolan is not the medium's only poster boy – it is symbolic that the new Star Wars trilogy, 15 years on from Attack’s groundbreaking digital filming, is now being shot on film once more. This summer, Dunkirk may well be seeing the biggest rollout of a 70mm presentation in cinemas for 25 years, but in 2015 Quentin Tarantino’s The Hateful Eight saw chains and independent cinemas having to retrofit 21st Century cinemas for a 20th Century presentation style. It was a difficult process, with only a handful of screens able to show the film as Tarantino intended – but it was a start.

Today, celluloid is, ostensibly, looking healthier. A recent deal struck between Hollywood big wigs and Kodak has helped. Kodak will now supply celluloid to Twentieth Century Fox, Disney, Warner Bros., Universal, Paramount and Sony. It’s a deal which is not only helping keep Kodak afloat, but also film alive.

Kodak has also gone a step further, launching an app to help audiences find 35mm screenings in local cinemas. Called ‘Reel Film’, it endeavours to back Nolan and co in ensuring that celluloid is still a viable method of film projection in the 21st century.

Even so, whether Nolan’s film fightback has actually had any impact is unclear. Independent cinemas still screen in film, and certainly Vue and Odeon both have film projectors in some of their flagship screens, but digital dominates. Meanwhile, key creatives are pushing hard for a digital future: Peter Jackson, James Cameron and the creative teams at Marvel are all pioneering in digital fields. Whether or not film can survive after over a decade of effacement is a difficult – and surpisingly emotionally charged – question.

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Paul Vickery, Head of Programming at the Prince Charles Cinema in London, is the kind of person you might expect to talk all about how physical film is a beautiful medium, key for preserving the history of cinema. History, he tells me, is important to the Prince Charles, but it's a surprise when he saysfilm is actually more practical for their operation. Because not every film they screen has been digitised, access to old reels is essential for their business.

“If you completely remove film as an option for presentation as a cinema that shows older films,” he says, “you effectively cut 75 per cent of the films that you could possibly show out of your options, and you can only focus on those that have been digitised.”

Vickery says the debate around film and digital often neglects the practicality of film. “It's always focusing on the idea of the romance of seeing films on film, but as much as it is that, it's also to have more options, to present more films. You need to be able to show them from all formats.”

That’s a key part of what makes the Prince Charles Cinema special. Sitting in London's movie-premier hub Leicester Square, the Prince Charles is renowned for its celluloid presentations of older films and has made a successful business out of its 35mm and 70mm screenings of both classics and niche films.

“If there is the option to show film and digital, we tend to take film as the option because it's also something you can't replicate at home,” he explains. “It's also just the nature of how film is seen on screen: its image clarity, its colour palette, the sound is just something that's very different to digital, and I think that's something that's very worth saving.

“Not many people have 35mm projectors at home. If you have it on Blu-Ray or DVD, to see it on film is a way of dragging someone out from their house to come and see it at the cinema.”

Currently screening is Stanley Kubrick’s 1968 epic 2001: A Space Odyssey in 70mm. It’s an incredible presentation of what Vickery says is a seven or eight year-old print struck from the film’s original negatives: the colour of the picture is far richer, while the fine detail in some close-up shots is on par with modern movies. Even more impressive, though, is that the screening is packed. “Fifteen years ago, there would be cinemas where that would be almost on a circuit,” laments Vickery. “We've just stayed the course, and that's something that's just fallen away and we're one of the last, along with the BFI, to show films from film.

“There’s still a bit kicking around, but as we do more and more of it, we seem to be pulling out those people who are looking for that and they seem to be coming back again and again. The repertory side of our programme is more popular than ever.”

That popularity is seemingly reflected in its audiences’ passion for celluloid. Vickery tells me that the PCC’s suggestions board and social media are always filled with requests for film screenings, with specific questions about the way it’s being projected.

For Vickery, it’s a mark of pride. “It sounds like inflated ego almost,” he begins, as if providing a disclaimer, “but it's why I think the work we do and the BFI do and any cinema that shows films from film is about history. By us continuing to show film on film, studios will continue to make their film print available and keep them going out. If people stop showing films on film, they'd just get rid of them.

“Once they're all gone, they only way we're ever gonna be able to see them is if they're taking these films and digitising them, which as you imagine, is always going to be the classic set of films, and then there'll be very select ones will get picked, but it's not gonna be every film.

“You have to keep showing films from film to keep the history of cinema alive in cinemas.”

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History is something that the BFI is committed to preserving. 40 per cent of their annual programming is projected on celluloid, and they loan around 200 prints to venues each year. Their new “BFI 2022” initiative will produce 100 new film prints in the next five years.

Most recently they have focussed on safeguarding their archive, the BFI’s creative director Heather Stewart tells me when we meet her in her office in the BFI’s artsy offices just off Tottenham Court Road.

“We got money from the government to renew our storage which was a big deal because the national collection really wasn't safe,” she says  “There was work at risk because it was warm and humid and we have bought a fantastic, sub-zero state of the art storage facility in Warwickshire in our big site there and our negatives are there. So our master materials are all in there safe - all the nitrate negatives and all that. In 200 years, people will be able to come back and make materials from those, whether digitising or analogue.”

Stewart tells me that it’s important to do both: “Do we at the BFI think that audiences need to see films in the way the filmmaker intended? Yes. That's not going away - that's what we're here for. Do we want as many audiences as possible to see the film? Yes. So of course we're interested in digital.”

The restoration and printing project is attracting lots of “international interest” according to Stewart: just one example is that the BFI are looking into partnering with Warner Bros in their labs in Burbank, California.

“We're becoming the only place left that actually loans film prints around the world so that you can see the films the way they were intended,” she says. “So if you don't have any kind of renewal programme, you'll eventually just have blanked out, scratchy old prints and you can't see them."

They're getting financial support too, she says: “There are people like Christopher Nolan, Quentin Tarantino, Paul Thomas Anderson [director of Oscar-winner There Will Be Blood whose 2012 film The Master was shot and screened in 70mm], a lot of people who are very committed to film, and so there's conversations going on elsewhere and with the film foundation about bringing other investments in so we can really go for it and have a fantastic collection of great great 35mm prints for audiences to look at.”

As a fan of the film reel, Stewart is passionate about this. I put to her the common suggestion that lay audiences can’t tell the difference between screening on film, and digital. “I don't agree with that", she says. "If you sit with people and look at it, they feel something that you might not be able to articulate.

“It's the realism the film gives you - that organic thing, the light going through the film is not the same as the binary of 0s and 1s. It's a different sensation. Which isn't to say that digital is 'lesser than', but it's a different effect. People know. They feel it in their bodies, the excitement becomes more real. There's that pleasure of film, of course but I don't want to be too geeky about it.”

Yet not every film print available is in good condition. “There's a live discussion,” says Stewart. “Is it better to show a scratched 35mm print of some great film, or a really excellent digital transfer?”

There’s no neat answer.

But Stewart is certainly driven by the idea of presenting films as closely as possible to the filmmakers’ true vision. “If you're interested in the artwork,” she explains, “that's what the artwork has to look like, and digital will be an approximation of that. If you spend a lot of money, and I mean really a lot of money, it can be an excellent approximation of that. But lots of digital transfers are not great - they're cheap. They're fine, but they're never going to be like the original.”

The process of restoration doesn’t end with digitisation. Keeping film copies in order to have originals is hugely important given how quickly digital media change. Film is a constant form of storage which does not alter. As Stewart defiantly puts it, “all archives worldwide are on the same page and the plan is to continue looking after analogue, so it ain't going anywhere.”

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The BFI were kind enough put on a display of how film projection works in practice. Tina McFarling, Media Advisor, and Dominic Simmons, Head of Technical, provide a tour of two screens at BFI Southbank. Chatting in the projection room above the screen which hosted the 70mm première of Dunkirk, their passion for celluloid was on display.

Standing next to two mammoth 70mm projectors, Simmons talks through the real-terms use of film, and the technical expertise behind it. “It's a lot more labour intensive than sticking digital prints on, but it's something we want to do,” he says.

One of the projection booths at the BFI

During the visit, the team are prepping a rare 35mm screening of the documentary I Am Cuba to be shown that afternoon. Simmons says that operating a celluloid projector is a “more complex operation” than digital. Looking at the endless labyrinth of film and sprockets, it's easy to believe.

“If you're screening from film in a cinema,” he says, “then you need engineers, technicians who are capable of doing it, whereas a lot of multiplexes have deskilled their operation.”

Simmons says that, while larger chains have one engineer to oversee every screen with the actual process of running the films centralised with a centre loading playlists, the BFI has twenty-two technicians, each closely overseeing the projection of a film when on duty.

“There's so much about the different elements of the presentation that you need to know that all comes together with the sound, the lighting and the rest of it.

“When you're starting a film, it's more of a manual operation. Someone needs to be there to press the buttons at the right time, manage the sound, operate the curtains, and attach the trailers to the feature.”

Having skilled operators is all very well, but of course you need to have the equipment to operate in the first place. “We have to make sure that the equipment is kept and utilised as well as making sure the prints are available, and then the skills will follow”, he says.

Simmons says many are likening the film fight back to vinyl’s resurrection, but has a rueful smile when he talks about film being described as “hipsterish” and “boutiquey”.

He also points out that the quaint touches that make film attractive to this new, younger audience – blemishes, the occasional scratch – are a headache for projectionists. “For me,” he says, “that's quite difficult because a bad print of a film is never a good thing, but if it's a bad print of a film that can't be seen any other way...” He trails off sadly.

The threat of damage to film prints is constant, he says. “Every time you run a film print through a projector there is some element of damage done to it. You're running it over sprockets at loads of feet per second.”

He switches a nearby projector on – it’s loud, quick and, after leaning in to look more closely, it’s easy to see that it’s violent. “It's a really physical process,” Simmons continues. “The film is starting and stopping 24 times a second.”

The idea that shooting on film, for which the very raw material is in short and ever-decreasing supply, is endangered is a tragic one. “There's a finite amount,” Simmons says. “People aren't striking new prints, so if you damage a print, the damage is there forever.”

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The Prince Charles and the BFI are in a privileged position to protect endangered film stock. A friendly partnership between them, which sees the BFI lending reels to the Prince Charles, as well as benefitting from the business of London’s rabidly cinephile audience, allow them to prioritise screening on film the majority of the time. Not every cinema is so lucky.

While the historic Ultimate Picture Palace in Oxford does have a 35mm projector, owner Becky Hallsmith says that it’s mainly the digital projector in use “for all sorts of logistic reasons”.

Though Dunkirk’s push for film projection was a welcome one, it still didn’t make sense for the UPP to screen it. “Certainly we thought about it, but I felt that if you're going to see it on celluloid, you probably want to see it on 70mm, so we decided not to get it on 35mm.”

Economic factors come into effect here too – the UPP, based just out of the city centre in Cowley, vies for Oxford’s filmgoers’ love with the Phoenix Picturehouse in nearby Jericho. While they do have slightly different markets, Hallsmith was aware that the Picturehouse was already set to screen Dunkirk in 35mm, leading her to decide not to.

 “It's not like I'm saying we never do it” she clarifies. “But there are reasons I haven't this time.”

Hallsmith was also aware that not all of her projectionists are trained in screening film, saying that, by screening Dunkirk in digital, she was “taking that little headache out of the equation”.

For the UPP, practicality of this kind trumps sentiment, given the cinema’s small operation. “I'd love it if I had the time to work out what films had beautiful 35mm prints and programme accordingly,” she says, “but I just don't have the time to put that amount of thought into details of programming. We're tiny. I'm doing all sorts of different jobs around the cinema as well. The programming is by no means the least important - it's the most important part of the job - but there is a limit to how much one can do and how much research one can do.”

Despite the practical issues related to 35mm, Hallsmith is still glad to have the option available, saying that when the digital projector was installed in 2012, there was enough room for the installation to account for the 35mm one – and to revamp it.

Despite many 35mm projectors being sent to an unceremonious death in skips, some projectors that are replaced for digital successors are cannibalised for parts. Hallsmith was a beneficiary. “Most of the bits on our 35mm projector are quite new,” she explains, “because they had all this stuff that they were taking out of other cinemas, so they upgraded our 35mm for us because they had all the parts to do it with.”

But Hallsmith is grounded when I ask her if having both projectors in operation is important. “It's important for me,” she laughs. “One of my real pleasures in life is to sit at the back near the projection room and to hear the film going through the sprocket. It's one of the most magical sounds in the world and always will be for me.

“But I know that for a lot of our customers, it is neither here nor there, so I have mixed feelings about it. It's not like I think everything should be on 35mm. I love it, but I can see the practicalities.”

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It is certainly practicality that’s governing cinema chains. Cineworld, Odeon and Vue have all seen huge expansions in recent years. Vue chief Tim Richards, says celluloid is a “niche product”, but the admission is tinged with sadness.

“The problem that we had,” he says about the 70mm screenings of Dunkirk, “with the conversion to digital that happened globally, there are literally no projectors left anywhere, and it's very, very hard to get one. We managed to find a projector and then we couldn't find anybody who actually knew how to run it. There are very real practical issues with the medium.

“To reinforce that we have a new look and feel to our head office, and I really wanted to have an old analogue 35mm projector in our reception and we couldn't find one. We had thousands of these things, and we had none left. We couldn't even get one for our reception!”

Even with a working projector and a trained projectionist, Richards says the format has “very obvious issues” with mass consumption. Again on the subject of Dunkirk, this time in 35mm, he says, “One of the prints that arrived was scratched. It's something that's been in the industry for a long time. If you have a big scratch, you simply can't screen it. You've got to get another print, especially when it will run through part of the film.”

It’s something that saddens Richards, who still says that projecting on film forms part of the “philosophy” of Vue. “We’re all big supporters [of film] and we love it. We've all been in the industry for between 25 and 30 years, the whole senior team. We genuinely love what we do, we genuinely love movies.”

That said, Richards, who is a governor of the BFI, is firmly committed to refining digital, more practical for Vue’s multiplexes. “If you go down and look at what we opened up in Leicester Square, our new flagship site, it's a 100 year old building where we shoehorned in new technology so it's not perfect, but it gives you an idea of what we're doing."

The new site has two Sony Finity 4K resolution projectors working in tandem – as well as the brand new Dolby Atmos sound system. The dual projection gives the screen a brighter, deeper hue. From a digital perspective, it is bleeding edge, and the set up is being rolled out across the UK and Germany, with 44 sites and counting. Richards is, as you would expect, enamoured with the results, claiming “that screen stands up to anything in the world”. What might be more surprising are the reactions he claims that it has elicited from celluloid devotees.

“There were a lot of old hardcore film fans there who were pleasantly surprised at the quality” he says. “People think of digital as being that new, TV-at-home which has got that clinical feel to it, and they don't feel it's got that warmth and colour saturation. This [Finity presentation] has that warmth of an old 35mm or 70mm, so I don't think the future is going back.”

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For Richards and Vue, the future appears to be as bright as that 4K Sony Finity screen in Leicester Square - for celluloid, not so much. While the appetite for watching movies on film might be growing at a promising rate for indie exhibitors, the list of technical and logistical problems is still insurmountable for many smaller venues - saying nothing of the race against time to preserve easily-damaged prints.

The main concern is an ephemeral one: the preservation of the knowledge needed to run a film projection. When the BFI’s Dominic Simmons speaks about the skills of his team and the need to pass those skills on, it evokes near forgotten skills such as thatching and forging. If the BFI and the PCC have anything to say about it, those projection skills will live on, but it’s unclear how far their voices can carry in a digital multiplex age.

As for the voice of celluloid-lover-supreme Christopher Nolan, even he too is shouting down what seems to be an unstoppable march towards a convenient digital future. But in a groundswell of growing interest and passion for the film reel, it seems that a director so obsessed with playing with time in his films seems to have bought exactly that for celluloid. Time is running out on the film reel, but there might be more of it left than we thought.