When listing special-effects-heavy films of the past 12 months, one that doesn’t exactly leap to mind is Suffragette. Sarah Gavron’s period drama is about the political awakening of a washerwoman called Maud, played by Carey Mulligan, in the early 1900s: in short, no teeming Orc armies, no tracking shots of homing missiles, and no sequences in which the then-Chancellor of the Exchequer David Lloyd George transforms into an articulated truck.

But even so, it contains just shy of 200 shots that have been digitally enhanced, if not effectively built from scratch, inside a computer – an average of two per minute. If you saw Suffragette and didn’t notice them, that’s because you weren’t supposed to. The best special effects in the film business today are the ones the business does its best to keep secret.

On an unglamorous Soho side street in the West End of London, between a youth hostel and a tattoo parlour, is the nondescript office block where Union VFX has made its nest. The boutique studio, co-founded by Tim Caplan and Adam Gascoyne, has quietly spent the past eight years building a reputation for what are known in the trade as “invisible effects”: the dramatic alteration of landscapes, sets and even actors in ways that are undetectable in the finished film.

Digitally enhanced: Suffragette contains several digital crowd-scenes Credit: Pathe

In Suffragette, that involved recreating England of the 1910s in all its hectic glory within the constraints of the film’s tight £9 million budget. Take the extraordinary establishing shots of Epsom Downs Racecourse on the day of Emily Davison’s death in protest, with thousands of spectators milling around and pressing in close to the track. In the original version of the footage, the course itself (actually Royal Windsor) is empty.

Each person in the crowd is a CGI model, built from full-body photographs of a small number of extras in period costume and animated by computer. The same technology also provided tents, cars, market stalls, bookmakers and a fairground with merry-go-round and helter-skelter.

In context, the shot looks thrillingly busy, and not remotely faked. That’s partly down to the photorealistic quality of the work, and also because of its plausibility. Photographs of enormous crowds in Edwardian Britain, including those in the newsreel footage shot on the day of Davison’s death, look exactly like this.

But it’s also because the effects themselves are completely ignored. In more obviously CGI-driven films, like Peter Jackson’s Hobbit trilogy, the camera circles and gawps at the artists’ handiwork. But Suffragette’s camera remains starchily rooted to the spot – just as it would have been out of necessity had the scene been shot for real. We’re now so used to the visual language of special effects that we can’t spot them without its help.

Suffragette is full of stuff like this: streets and buildings that don’t exist, vehicles that were long since scrapped, smashed panes of glass that never broke. The effects business is “no longer about every shot being an exploding building”, Caplan tells me. Instead, its job is to make “stories sit in the real world” – which involves bending the real world into shape.

The rationale behind using invisible effects is almost always cost. “Creating Forties New York out of Liverpool makes sense when a film’s budget barely gets the crew to Liverpool,” says Caplan. Convenience comes into it too: digital snow is easier to work with than the real thing

In Robert Zemeckis’s The Walk, it made less sense to train Joseph Gordon-Levitt to juggle on a tightrope than it did to build a 3D mask of the actor’s face and paste it on a professional tightrope-walker. Sometimes Gordon-Levitt is actually on the wire, and sometimes he isn’t even on screen.

Joseph Gordon-Levitt in The Walk Credit: Sony

Car manufacturers got in on it a long time ago. Virtual vehicles are easier to light and keep clean, and don’t reflect camera crews, which is why most car ads haven’t featured real cars for years.

As well as Suffragette, Union VFX has recently provided invisible effects for Danny Boyle’s Steve Jobs, Stephen Frears’s Philomena, Thomas Vinterberg’s Far from the Madding Crowd, and James Marsh’s The Theory of Everything. And their work takes in all the “traditional” aspects of film-making craft: location-scouting, production design, cinematography, make-up, and even performance itself. Eddie Redmayne won an Oscar and a Bafta for his embodiment of the young Stephen Hawking in Marsh’s film, but some of that was only made possible with digital help. A haunting shot of Redmayne’s wrist muscles spasming involuntarily was achieved by combining two separate images of his hand: one in which he wiggled his fingers (which provided the muscle movement) and the master shot in which he kept them still.

Caplan and Gascoyne don’t see this as trickery. To them, the performance is still all Redmayne: they just helped piece it together. Gascoyne says that they’ve “yet to encounter” an actor or crew member who’s suspicious of their trade: “They all recognise what we do is in service of the story,” he says.

Nevertheless, invisible effects are rarely nominated for Oscars or Baftas – awards that Caplan describes, without a trace of envy, as being designed to “celebrate spectacle”. Instead, they crop up in other categories, completely unacknowledged. The Wolf of Wall Street’s beachside mansions? Faker than their owners. Argo’s vision of Tehran 1979? Istanbul plus CGI. Every single eye-narrowing snowstorm in The Girl with the Dragon Tattoo? Painted onto clear skies. The Revenant’s angry bear? As incorporeal as Yogi.

“Among our peers in the film industry, you get a sense for how they feel about our work. The word ‘seamless’ is the biggest compliment we get. When we hear that from our clients, that’s the buzz,” he says.

The most seamless aspect of invisible effects work is also the most secretive: so much so that it’s routinely carried out on a need-to-know basis and under confidentiality agreements. It goes by a number of euphemistic names – “beauty work” is one of the prettier ones – but it’s effectively digital cosmetic surgery. Two things to know about this: almost every mid to high-budget film you see now involves this procedure as a matter of course, and no one on the cast or crew is legally permitted to talk about it.

Beauty work has its roots in music videos in the mid-Nineties. Caplan remembers carrying it out himself back then at the venerable effects house Cinesite, “getting rid of the bags under the artists’ eyes, or giving Tina Turner a tummy tuck”. The glossy, glamorous look favoured by artists of the day – think Britney Spears and Christina Aguilera – masked the technical shortcomings. But around 10 years later, the software became subtle enough for the film industry, and the shrewder studios discreetly brought it into their repertoires.

At first it was used as a corrective, and only where required. During the making of the 2005 thriller The Jacket, the usually radiant Keira Knightley suffered from an outbreak of bad skin, so effects artists developed a piece of software, code-named “Keira-sil”, that digitally smoothed things over.

Keira Knightley, whose skin was enhanced with digital effects, with Adrian Brody in The Jacket Credit: Warner/Everett / Rex

But now, anything’s possible – necessary or otherwise. Teeth are straightened, lips plumped, breasts and bums lifted, bald spots covered, jawlines squared off, heads pasted onto body doubles. Actors and actresses in their 30s, 40s and 50s turn to it instead of real-life plastic surgery – and occasionally to counteract the deadening effects of it. Younger stars are having their bodies scanned in minute detail as source material for invisible retouching in the years to come.

The market has been cornered by a few small, Los Angeles-based studios that don’t advertise much and are wary of outside attention. One of the best, Vitality Visual FX, has nothing on its website bar a logo and an email address. Another, Lola VFX, is less circumspect. Until recently, its site offered “digital weight management”, “six-pack abdominals”, “eye enhancements”, “breast augmentation” and “skin resurfacing”, among other services. But you won’t find many examples on its showreel.

The ones they’re open about are the ones you’re meant to notice. It was Lola that de-aged Patrick Stewart and Ian McKellen by 20 years in X-Men: The Last Stand – and provided the same service for Brad Pitt in The Curious Case of Benjamin Button.

A de-aged Michael Douglas in Ant-Man Credit: AP / Paramount

This summer, it took 71-year-old Michael Douglas back to his Fatal Attraction vintage for the prologue to Marvel’s Ant-Man.

I’ve noticed what I suspect has been substandard beauty work a handful of times over the past few years: mostly in mid-budget comedies and thrillers with A-list casts but without the means for meticulous retouching.

But most of the time, it’s impossible to spot, and I’ve given up trying. The elusiveness of film’s connection to the real has always been one of its most seductive charms.

Now it’s just able to trick our eyes one more way.