Flange focal distance is one of the most important variables in a system camera, as lens seating errors of as little as 0.01 mm will manifest themselves critically on the imaging plane and focus will not match the lens marks. Professional movie cameras are rigorously tested by rental houses regularly to ensure the distance is properly calibrated. Any discrepancies between eye focus and measured focus that manifest them­selves across a range of distances within a single lens may be collimation error with the lens, but if such discrepancies occur across several lenses, it is more likely to be the flange focal distance or the ground glass (or both) that are misset.

Liquid lenses offer several advantages over traditional lens technology. The main one is their ability to alter focal length without any moving mechanical parts. That means liquid lenses can be kept small and light.

Not so with liquid lenses: they use a small pocket of fluid instead of physical discs, changing shape to direct and focus light. They’re small, super-fast and sturdy, too. And they could change the world of smartphone photography.

The Edmund Optics M12, for example, is a lens recommended for use in production systems that require high-speed machine vision. This includes assembly lines, where objects of different sizes are inspected as they roll out of the plant. With all refocusing controlled by the liquid lens, the M12 is able to rapidly adjust to items as they pass beneath the sensor.

As any owner of a DSLR or mirrorless camera will know, no single lens is fit for every situation. Some can cover fairly large zoom ranges, but they have to be sizable simply because of the space required to fit the optical elements inside. And if a lens doesn’t cover the focal length appropriate for a given scene, you’ll need to switch to different glass – as long as you have some stashed in your camera bag.

In general terms, when a camera’s electric focus motor fires – or you manually twist the tube – the physical distance between those optical components changes. This in turn changes the focal length, bringing the image into focus on the sensor or, in the case of a zoom lens, altering the field of view – from wide-angle to telephoto.

Liquidcrystal lens

Other solutions deploy alternative approaches to achieve the same result. Optotune’s focus-tunable lenses, for example, seal the optical liquid in a polymer membrane sac, which is then manipulated using electrical current.

How Xiaomi’s approach compares isn’t yet clear: we haven’t been able to get hands-on with the China-only handset. Theory suggests a physical motor will be slower than a focus system which uses electrical current alone, but the liquid core will still deliver new focal length flexibility.

While that’s fine for the sensors found in most compact cameras, it doesn’t come close to covering Micro Four Thirds or APS-C sensor sizes. Until the technology grows, liquid lenses are unlikely to replace traditional interchangeable glass. So it’s worth hanging onto your camera bag for the time being.

One technique – known as ‘electrowetting’ – pairs water with a separate layer of non-conductive oil, which sit together in the lens. By applying voltage across the boundary between the two, the curvature – and so the refractive effect – can be altered in a fraction of a second. Corning’s Varioptic lenses use this technology.

For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), back focus[1] or register, depending on the usage and source) of a lens mount system is the distance from the mounting flange (the interlocking metal rings on the camera and the rear of the lens) to the film or image sensor plane. This value is different for different camera systems. The range of this distance, which will render an image clearly in focus within all focal lengths, is usually measured to a precision of hundredths of millimetres, and is not to be confused with depth of field.

The liquid lens concept has been implemented in several ways, with different methods employed to exert pressure on the capsule. But the magic almost always happens when electricity is added.

Once the potential for liquid lenses is proven, though, it’s not far-fetched to suggest we may see a return to single lenses on future smartphones. If a single snapper is capable of switching quickly between focal lengths, zooming optically and resisting vibrations, why add more?

Tunable lens

Most liquid lenses use no mechanical components. Instead, they contain a transparent fluid capsule. By applying pressure to part of this pod, its shape can be manipulated. Squashing or stretching it alters the trajectory of light passing through the optical-grade fluid – and therefore the focal length.

Liquidlens edmund optics

Formerly News Editor at Stuff, Chris now writes about tech from his tropical office. Sidetracked by sustainable stuff, he’s also keen on cameras, classic cars and any gear that gets better with age.

Lenses can be adapted from one mount (and respective FFD) to another. FFD determines whether infinity focus can be accomplished with a simple non-optical adapter. Optics to correct for distance introduce more cost and can lower image quality, so non-optical lens adapters are preferred. A simple non-optical adapter holds the longer FFD lens the appropriate additional distance away from the sensor or film on the shorter FFD camera. A camera body with a shorter FFD can accept a larger number of lenses (those with a longer FFD) by using a simple adapter. A lens with a longer FFD can be more readily adapted to a larger number of camera bodies (those with a shorter FFD). If the difference is small, other factors such as the sizes and positions of the mounting flanges will influence whether a lens can be adapted without optics.

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Perhaps understandably for an experimental imaging system, Xiaomi included standard cameras alongside the liquid lens on its Mi Mix Fold. Many manufacturers are likely to follow suit: in a smartphone market where multiple lenses are the norm, there’s a good chance you’ll see brands adding liquid lenses as one of many on their mobiles – at least until they understand the true capabilities of the technology.

But it seems highly likely that liquid lenses will be deployed more widely in the smartphone market. If its patent filings are to be believed, Huawei already has plans to deploy the tech. Many predicted it would appear on the new Huawei P50 Pro powerhouse. It didn’t – but a liquid lens could yet show up on the P50 Pro+, which is rumored for launch later this year.

Because most liquid lenses don’t use electric motors, they can normally focus far more rapidly than their mechanical cousins. The optical liquid inside reacts almost instantaneously to electrical current, which results in super-fast focus performance, plus pinpoint accuracy. The option to switch focal lengths in mere milliseconds could open up a world of possibilities for multi-talented photographers, too. Spotted a rare bird in the distance while shooting a flower in macro? No problem.

Despite their availability and potential advantages, liquid lenses have only just arrived on smartphones. Xiaomi’s folding Mi Mix Fold flagship – launched in April 2021 – is the first consumer handset to feature a liquid lens.

Deep optics

The average camera phone lens today is restricted to a fixed focal length. Adding versatility means either adding more lenses – or switching to a larger camera with adjustable optical elements.

While their size makes liquid lenses ideal for use in smartphones, it’s trickier to leverage the technology for larger camera formats.

Just like prime lenses, most standard smartphone cameras feature a fixed focal length, thanks to their static optical elements. It’s rare to see genuine optical zoom on a smartphone, because it requires separation of these elements; even at microscopic levels, this is difficult to achieve within the slimline confines of modern mobile design. And that’s where liquid lenses come in.

Focus speed is just one aspect of a camera’s performance. Even with near-instant focal length adjustments on offer, pro photographers will still want the low-light skills and outright image quality offered by bigger sensors. That poses a problem: according to Edmund Optics, the largest liquid lens aperture is currently 16mm, which means it’s only suitable for use with sensors up to 1/1.8in.

Imagine a water droplet on a leaf – the kind you often see captured on the pages of photography magazines. Just as this bead can refract natural light, so too can the fluid in a liquid lens guide light onto an imaging sensor.

Optotune

Tube lens

Thanks to their durability, tiny size and rapid reaction times, liquid lenses have been seen in industrial settings for several years.

Current limitations mean larger liquid lenses are likely a long way off. But they’re already available in a form which fits most mobiles. Paired with the most powerful sensors yet seen in smartphones, the versatility of liquid lenses could revolutionize pocket photography – and sound the final death knell for compact cameras.

Traditionally, the course of light through a camera lens is controlled by a series of polished optical elements. These curved discs – usually made from glass or perspex – are used to ‘bend’ light as it passes down the barrel. With the correct spacing between lens elements and the sensor, an image should arrive at a single fixed point, sharp and in focus.

Like a standard smartphone lens, the physical aperture of a liquid lens doesn’t change. But liquid lenses offer a spectrum of focal lengths. Those two facts together should mean greater depth of field flexibility for mobile photographers, as well as improved low-light performance – aided by the low dispersion and high transmittance of light through the optical fluid.

Typically, camera bodies with shorter flange focal distance can be adapted more readily to lenses with longer flange focal distance.

Their tiny dimensions make them ideal for use on smartphones. So, too, does the ability to switch quickly between focal lengths, which could put an end to the multiple lenses now sprouting up on the back of smartphones. And if liquid tech can be integrated into the larger lenses used by DSLR and mirrorless cameras, your arsenal of bulky barrels could potentially be replaced by a single fluid solution.

When liquid lenses use no mechanical components, durability is increased, as there are no moving parts to wear out or fail. Eliminating these elements also means liquid lenses consume minimal power, which should be good news for smartphone battery life. And whichever technique is used to manipulate the fluid cell, the liquid itself is generally less sensitive to vibration than a standard lens, which should result in improved optical image stabilization.

As above, Xiaomi’s system uses a high-precision electric motor to alter the capsule’s shape. At the time of writing, it’s the sole smartphone with a liquid lens – and it’s currently only available in China.

Due to research on optimal flange focal distance settings, it is currently considered better for flange focal distance to be set to somewhere within the film's emulsion layer, rather than on the surface of it. Therefore, the nominal flange focal depth will be equivalent to the distance to the ground glass, whereas the actual flange focal depth to the aperture plate will in fact be ~0.02 mm less.

Lenses and liquids are generally best kept separate, yet the future of photography could lie in combining the two with, you guessed it, liquid lenses.

Xiaomi’s liquid lens tech, seen for the first time on the Xiaomi Mi Mix Fold (see above), takes a third route. The refractive fluid is contained in a film membrane, with a precision electric motor accurately deforming its radius to adjust focal length. The result is a single lens with a minimum focus distance of 3cm for macro shots and an optical zoom range of 3x – expanding to 30x with the help of some in-device digital trickery.