Is the liquid lens on the Mi MIX folding screen a gimmick or a black technology?

After two consecutive conferences, Xiaomi has brought us many surprises, including the "Android King" Xiaomi 11 Pro, the "Android Light" Xiaomi 11 Ultra, the first folding screen MIX FOLD, and high-profile announcements to enter the field of electric vehicles.

"Made by Xiaomi" is just a declaration, there is no more information yet, so please click here first. Xiaomi's three mobile phones that hit the high-end market are actually the protagonists of this Xiaomi "Spring Festival Gala".

In retrospect, Xiaomi Mi 11 Pro and Xiaomi Mi 11 Ultra are actually more like "stacking kings", and MIX FOLD contains Xiaomi's "Tomorrow Technology", one of which is the "liquid lens", which is also MIX. A highlight of FOLD.

MIX FOLD puts the "lens" into the telephoto lens

In the summary article of the Xiaomi conference, I described MIX FOLD as the "Dinghai Shenzhen" in the Xiaomi Armory.

This way to describe MIX FOLD is not because it has an unconventional folding screen form, but because it has two "world premiere" technologies, one is "liquid lens" and the other is "surging C1". These two tomorrow's technologies are not only showing muscles, but also confirming the purpose of the MIX series "exploration".

For the function of the built-in independent ISP Surging C1, we may have a certain concept, or to make the image processing more accurate and fast. But the "liquid lens" may be a bit "unfamiliar". From an intuitive point of view, it is as if Xiaomi made the lens into a liquid, allowing the light to pass through the liquid to focus.

Many questions have arisen from this, such as how the uncontrollable liquid can accurately transmit light to the sensor, how to control the accurate deformation of the liquid to quickly move the focus position, and how to ensure the image quality of the liquid lens in extreme environments, and so on.

▲ Close-up of MIX FOLD liquid lens. Picture from: Xiaomi

Long before the official debut of MIX FOLD, Xiaomi had already released the principle and characteristics of the "liquid lens". It can be seen from the public information that the liquid lens on MIX FOLD mainly brings one of the biggest advantages, that is, the macro function is integrated into the telephoto lens. While supporting 30x zoom, it has a recent 3cm Focus distance.

Our human eyes are very precise and have a strong ability to focus. Through the control of the ciliary muscle to the lens, we can see very close or very far, but the angle of view remains the same.

The liquid lens on the MIX FOLD has the same characteristics as the "human eye", that is, it can "see close (macro)" or "see far", but the focal length (angle of view) is not changeable, otherwise the phone lens will have to "see" "Nine Nine into One". Only the transparent fluid wrapped in the film is simulated as a "lens", and the motor that controls the change of the liquid is similar to the ciliary muscle of the human eye.

After the transparent fluid in the lens receives the pressure of the motor, it can quickly change its shape and determine the focus to complete the focus. This process is theoretically faster and more stable than traditional mechanical devices. The liquid inside the lens, according to the official statement, will have very good light transmittance and resistance to extreme environments. Since energy is produced, there is no need to worry too much about the stability of the "liquid lens".

▲ The first one in the MIX FOLD camera module is the "liquid lens". Picture from: Xiaomi

As for the final "curative effect", I immediately asked Zhijian, who had experienced MIX FOLD on the spot, and he just said that there was no special feeling. And this feeling can be regarded as an affirmation of a new technology. After all, "the real technology makes you unable to feel its existence." As for whether the focus is faster and the quality is better, you may still have to wait for the true MIX FOLD. The machine has been tested, so stay tuned.

A liquid lens that is not a new thing, but it blooms in the moving image

Before the advent of Xiaomi MIX FOLD, some special "liquid lens" industries were quite mature, but they were not related to "images" and were not used for imaging and shooting, but more for scanning records.

The biggest application scenario for these special lenses is the scanning camera on the conveyor belt of the e-commerce automatic sorting center. They use the liquid lens to be able to quickly autofocus, and the focus range is large, which can quickly scan barcodes on products of different sizes. So it is recorded in the program to facilitate inventory management.

▲ The dynamic autofocus system responsible for scanning codes on the conveyor belt. Picture from: COGNEX

In addition, liquid lenses are also used in some microscopes to quickly and accurately focus on various object planes.

In general, in industry and some special fields, liquid lenses have been relatively perfect from manufacturing to use, and their principles are not much different from those of Xiaomi’s "liquid lenses". They are all driven to the edge of the film through a motor (current driver). Applying pressure changes the curvature of the electrofluid, and then completes the movement of the focus.

▲ A liquid lens that changes the curvature of the electrofluid to focus. Picture from: Edmund Optics

However, MIX FOLD is the first and the only one that applies the "liquid lens" to the outsole sensor and the moving image that requires high imaging requirements.

As early as 2010, major imaging manufacturers Canon and Sony had applied for patents similar to "liquid lenses", but until now they have not mass-produced or launched corresponding concept products. It may be because the "liquid lens" did not achieve the expected imaging after covering the full-frame image field.

Or for a full-frame camera with interchangeable lenses, the macro lens is originally a telephoto lens. The functions of the two can be achieved under the original mechanical structure, and there is not much market demand to urge them to continue. Innovation, so the "liquid lens" project was stranded.

On the other hand, in the field of mobile imaging, competition is much more fierce. Without continuous innovation, it is difficult to differentiate only by relying on components and algorithms provided by upstream suppliers, and it is also difficult to gain a foothold in this field, let alone. Want to continue to sprint to the high-end market. Perhaps the emergence of "liquid lenses" is just the beginning, and more manufacturers may bring other "tomorrow" technologies in the future.

What's the point of integrating macro into a telephoto lens?

The integration of the macro function into other lenses is not the first of Xiaomi MIX FOLD, but their directions are different.

Starting in 2018, a batch of new flagships represented by Huawei Mate20 have cut off the macro lens that was made up for by complaints. Integrate the macro function into the ultra-wide-angle lens.

This does not only rely on software algorithms, but more importantly, the customization at the hardware level, so that the ultra-wide-angle focusing distance can be as close as 2 cm, so as to achieve macro shooting.

Integrating the macro into the ultra-wide-angle lens not only simplifies the rear camera module, but also improves the mobile phone's ability to shoot macros. After all, the ultra-wide angle and resolution of high-pixel are better than the common macro lens of 2 to 5 million pixels.

Nowadays, ultra-wide-angle support for macro shooting is a common feature, and it is mature enough to be decentralized to mid-range models. Why does Xiaomi MIX FOLD have to spend a lot of time to come up with a "liquid lens" that combines a macro lens and a telephoto lens into one?

First of all, we have to understand the relationship between macro and telephoto lenses. Friends who know camera lenses know that macro lenses are actually telephoto lenses. For example, Canon's "Hundred Micro" lens and Sony FE 90mm lens are classic macro lenses.

▲ Taken with Canon Baiwei. Picture from: Ery Budi Nurhudha

From the perspective of focal length, a lens over 50mm can be called a telephoto lens. Can a telephoto lens be used for macro shooting? The answer is not necessarily. The core of the problem lies in the closest focusing distance.

The telephoto lens on the mobile phone is limited by the lens structure, and it is difficult to focus on the subject at close range. Believe it or not, turn on the telephoto lens and shoot with your finger. When the distance is too close, the phone will defocus or automatically switch to the main camera for digital cropping.

In order to achieve macro shooting with telephoto lenses, mobile phones such as Redmi K30 Pro and Xiaomi 11 have all customized their lenses so that the focus range of their telephoto macro lenses is limited to 3-7cm.

As a result, this telephoto lens can only shoot macros, not distant scenes. If you want to shoot far, you can only rely on the digital crop of the main camera.

▲ The telephoto macro lens of the Redmi K30 Extreme Commemorative Edition

The liquid lens of Mi MIX FOLD solves the focus problem of the telephoto lens of the mobile phone very well. Simply put, it can take clear shots at near and far.

And the purpose of Xiaomi's research and development of this lens is not just to sell concepts, display technical capabilities and establish differentiation. Because shooting macro with the telephoto end has many advantages over the ultra-wide angle.

First of all, due to the exponential increase of the focal length, shooting macro at the telephoto end has a greater magnification than the super wide-angle at the same position. The subject can be enlarged in the viewfinder.

▲ Picture from: Zhihu

Secondly, shooting macro at the telephoto end can achieve a better sense of compression and a shallower depth of field. In other words, the subject can be more prominent, the picture is more concise, and the background blur is better.

Not only can it save the position of a lens, but it can also achieve better macro shooting. This is the meaning of the current Xiaomi liquid lens.

How can mobile phones return to the "single-shot era"?

To be honest, when I first saw the poster of the Xiaomi liquid lens and heard about this concept, I thought it was using "black technology" to achieve continuous optical zoom.

There are many friends who have the same ideas as me. I thought that this technology could "eliminate" the numerous cameras on the back of the mobile phone, and use a single lens to achieve full-focus shooting from wide-angle to telephoto. But later I found that the imagination was too good, and the liquid lens was mainly used to solve the focusing problem.

So how can mobile phones return to single shots while preserving the current photography experience? This requires the idea of ​​using a camera zoom lens. For example, the Asus Hawkeye ZenFone Zoom released in 2015. The single lens behind it can achieve three times optical zoom (28mm-84mm).


Although it achieves continuous zoom, the thickness of the lens module is still controlled within 12mm. The secret lies in the structure of the periscope. ASUS placed the lens module parallel to the fuselage and refracted the scene into the sensor through two sets of prisms, solving the problem of fuselage thickness.

Then through the motor of the lens module, the lens is controlled to move back and forth to achieve continuous optical zoom.

This sounds like a great solution. But what about the product experience? Our biggest feeling at the time was the slow imaging speed, high shutter delay, and the picture quality can only be described as unsatisfactory.

If you put this solution today, perhaps with the support of outsole sensors, more advanced SoCs, and computational photography, the image quality problem can be effectively solved. It can be said that it was born in the wrong era.

It is worth mentioning that the shortest focusing distance of this phone at the wide-angle end is 5cm, and when using the telephoto end, the shortest focusing distance is 30 cm. If liquid lens technology can be applied, perhaps it can also achieve 5cm macro shooting at the telephoto end.

Similar to the ASUS ZenFone Zoom solution, vivo brings a lens that can achieve 5-7.5x optical zoom on APEX 2020. It has built-in two-group fixed and two-group movable "four-group lens combination" to ensure high image quality under continuous zoom.

In order to achieve a body thickness of 8.8mm, the battery capacity of APEX 2020 is only 2000mAh. It can be seen that the continuous optical zoom lens module is so large that it is currently difficult to apply on a large scale.

▲The zoom experience of APEX 2020

In June last year, OFILM announced that it had developed the thinnest periscope continuous zoom module in history. The thickness is only 5.9 mm.

The lens of this module is composed of 3 groups of "1GMO6P", the equivalent focal length reaches 85~170mm, and the aperture is F3.1~5.1. If the focal length of the main camera is 28mm, this lens is equivalent to a continuous optical zoom of 3-7 times.

However, if the periscope zoom module is to be applied to mobile phones on a large scale, there are still many ways to go. Such as imaging quality, difficulty of assembly, cost and so on.

But this kind of scheme still makes people look forward to. Perhaps in the near future, we can see a mobile phone equipped with a liquid lens + periscope zoom module. When the two "black technologies" are combined, a new world will usher in the back of the phone.

Author: Du Yuan best man, get a new word

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