Mobile phone cameras have shown a distinct trend in the past two years: the super wide-angle is getting stronger and stronger, and the super-telephoto scene is no longer.
Observing the flagship machine in 2021, it is not difficult to find this pattern. Most mobile phones either directly castrated the ultra-telephoto lens, or shrunk the related configuration. The ultra-wide-angle has received more attention than before: micro-pan heads, ultra-outsoles with the same specifications as the main camera, and special free-form lenses are appearing on more and more mobile phones.
Thanks to the blessing of these new technologies and new hardware, the picture quality of the ultra-wide-angle lens of mobile phones has ushered in a qualitative leap.
But compared to mainstream sensors such as IMX766 and IMX586, "free-form lens" is still a fuzzy concept for most users. It is still exclusive to a handful of flagship phones such as Huawei Mate40 Pro+, OPPO Find X3 series, OnePlus 9 series and so on.
In terms of controlling distortion, is the free-form lens really as magical as in the publicity? What is its realization principle? Will it affect the perspective of ultra-wide angle? These will be the topics discussed in this article.
Where does lens distortion come from
Before understanding the free-form lens, we need to know how the picture distortion is produced.
First of all, it must be clear that lens distortion is a kind of aberration and has no effect on the clarity of the picture. It's just that the distorted objects affect the perception and lose the realism.
For example, in the picture below, the building on the left is obviously deformed, and it seems that the ground is not flat. The corrected picture on the right is close to what the naked eye actually sees.
▲Picture from: ehabphotography
Why does distortion occur?
We know that a lens is composed of several lenses (lenses). The light passes through the various lenses and finally reaches the sensor, thus forming an image.
▲ Picture from: Leica Rumors
However, the magnification of the lens changes as the angle between the beam and the main axis changes. Simply put, when the light passes through orthogonally to the main axis, there is generally no distortion. The farther the light is from the main axis, the greater the angle when passing through, and the more obvious the distortion will be.
Almost all lenses cannot avoid distortion. Common distortions can be divided into two types: barrel distortion and pincushion distortion.
As the name suggests, barrel distortion will cause straight lines to bulge toward the edge, which looks like a wine barrel, which is more common in wide-angle lenses. Pillow distortion will cause the straight line to dent inward, resembling a pillow, which is more common at the telephoto end.
For example, when we take a group photo with a wide-angle lens, barrel distortion often occurs, causing the non-central face to be stretched and deformed, and it looks like a circle of fat.
▲ The left is a diagram of barrel distortion, and the right is the appearance after removing the distortion. Picture from: dpreview
In the traditional photography era, the method of eliminating distortion is mainly to use photo editing tools such as PS to correct the distortion. This can often achieve good results.
▲Using PS for distortion correction. Picture from: youtube
But in the era of mobile photography, people want to simplify the photography process and reduce the cumbersome post-retouching. So mobile phone manufacturers began to delve into the algorithm to eliminate the distortion, trying to automatically correct the distortion when the film is formed.
▲Google anti-distortion algorithm display. Image source: Google
Until the launch of Huawei Mate40 Pro+ last fall, it provided a new idea for anti-distortion of the ultra-wide-angle lens of mobile phones.
Principle of free-form lens
We mentioned above that the distortion is mainly caused by the difference in the magnification of the lens system. So improving the design of the lens from the physical level is undoubtedly an effective solution.
How to improve it? The answer is free-form surface design.
If you have high myopia, you should have heard of free-form lenses when you wear glasses. By introducing non-axisymmetric, irregular, and complex curved surfaces, free-form surface lenses get rid of the disadvantages of traditional lenses such as narrow field of view, distortion and blurring around the height of the lens.
▲ Zeiss free-form lens. Picture from: Carl Zeiss
This kind of lens is between the spherical surface and the aspherical surface, and the optical principle involved is very obscure. What we need to know is that its irregular and free curved surface allows beams of different angles to pass through the main axis while maintaining approximate magnification and minimizing aberrations.
▲Picture from: cocoleni
To put it simply, glasses users can use free-form lenses to see a broader and clearer vision. At the same time, the lenses are light and thin, which can be described as "a combination of multiple talents."
▲ ZEISS free-form lens advertisement
However, processing a high-precision free-form surface lens is complicated and costly. Generally exist in the high-end, customized glasses market. For example, Zeiss's high-end driving lenses and progressive lenses have free-form surfaces.
▲The manufacturing process of free-form lens
In fact, free-form surface lenses are not only used in the eyewear industry, but also in optical-related medical, projection, printing and scanning fields. The field of optical lighting is more widely used, and the free-form surface design can significantly enhance the uniformity of lighting.
It can be seen from the above description that the free-form surface lens has an excellent effect on reducing aberrations, that is, reducing distortion. For mobile phone manufacturers suffering from ultra-wide-angle distortion, it is undoubtedly a design worth learning.
However, due to the limited volume of the mobile phone camera module and the complicated stacking of internal components, high requirements are placed on the design and processing technology of the free-form surface lens.
And Huawei Mate40 Pro+ is the first "crab-eating mobile phone", bringing "the world's first free-form surface lens." According to its behind-the-scenes designer, Dr. Changlun Hou of Hangzhou Dianzi University, they began experimenting with free-form lenses in 2018 and after repeated verifications, they finally had the Mate40 Pro+ two years later.
In 2021, OPPO and OnePlus also successively brought mobile phones with free-form lenses, which were repeatedly promoted as major selling points. For example, OnePlus Liu Zuohu has repeatedly hailed the free-form lens on Weibo, saying that it can reduce the distortion to less than 1% and is the best partner for ultra-wide angle.
Judging from the propaganda data, it seems pretty good. However, the actual effect and whether the perception is strong or not, the proofs have the final say.
The effect of the free-form lens seems to be exaggerated
Take the OnePlus 9 Pro that I have used for more than two months as an example. If you take a camera with no doped algorithm as a reference, the ability of OnePlus 9 Pro to control distortion at an ultra-wide angle is obvious.
For example, in the picture below, the barrel-shaped distortion of the elevator door frame taken by the camera is very obvious, but the OnePlus 9 Pro does not have this problem.
▲ The left is a full-frame camera using a 24mm lens to shoot, the right is a OnePlus 9 Pro shooting
In this scene, the TV frame taken by the camera has already undergone significant distortion, while the picture of OnePlus 9 Pro still has straight lines.
▲The left is a full-frame camera using a 24mm lens to shoot, the right is a OnePlus 9 Pro shooting
So can it be considered that a free-form lens can completely eliminate barrel distortion?
The answer is not necessarily. One is that we are not sure whether the anti-distortion algorithm is involved or how much is involved in the ultra-wide angle of the OnePlus 9 Pro. In other words, all the credit for suppressing distortion is not necessarily due to the free-form lens. Second, the distortion is difficult to eliminate 100%, and can only be reduced to a certain limit, which is not easy to detect.
▲ OnePlus 9 Pro ultra-wide-angle shooting
Perhaps we can change our thinking and compare the OnePlus 9 Pro with a pure algorithm anti-distortion mobile phone, and it seems that we can know whether the advantages of the free-form surface lens are strong or not.
When compared with the Xiaomi 11 Ultra with the ultra-wide-angle distortion correction algorithm turned on, it can be seen that OnePlus 9Pro has clearly won. But this comparison is not fair, because the Mi 11 Ultra ultra-wide-angle focal length is wider, of course, the distortion will be more serious.
▲ Xiaomi 11 Ultra on the left, OnePlus 9 Pro on the right. Picture from: androidauthority
So how does it compare with the iPhone 12 Pro Max, which has a relatively close focal length? The answer may be unexpected.
Let's take a look at the photos before and after the distortion correction is turned on on the iPhone 12 Pro Max. In the same scene, the addition of the distortion correction algorithm makes the barrel distortion almost invisible.
▲The picture on the right shows the open state of the distortion correction algorithm
Compared with the OnePlus 9 Pro, even though the ultra-wide field of view of the iPhone 12 Pro Max is wider, with the excellent algorithm, its distortion is not obvious. It is difficult to say that the former is better at suppressing distortion.
▲iPhone 12 Pro Max on the left, OnePlus 9 Pro on the right
Another example is the following scene, the two are equally excellent in suppressing distortion, and even the pure algorithm iPhone 12 Pro Max is even better. The edge of the wall (indicated by the red frame) taken by OnePlus 9 Pro is slightly distorted.
Therefore, in the face of excellent algorithms, the advantages of free-form surface lens anti-distortion are not so obvious. From this perspective, the effectiveness of free-form lenses seems to be exaggerated.
Free-form lenses do not weaken the sense of perspective
The reason why people use ultra-wide-angle shooting is not only to record a wider scene by pressing the shutter, but also to pursue the unique visual enjoyment brought by the sense of perspective.
For example, if the lens is up to shoot trees or buildings, the subject seems to be about to fall down, which has a strong visual impact. This is called perspective distortion (also called linear distortion).
▲ Picture from: OnePlus Mobile
Although perspective is essentially a kind of deformation, this kind of deformation conforms to the laws of nature and what the human eye actually sees. To put it simply, it follows the rule of "near big, far small". It was widely used in painting art and later became an important photographic language.
So, will free-form lenses inhibit the performance of perspective?
The answer is no. Ultra-wide-angle lens with free-form surface lens can produce excellent suppression effect on barrel distortion, but will not eliminate perspective distortion.
If you want to eliminate perspective distortion, you can only place the lens parallel to the subject when shooting. It is unrealistic to count on free-form lenses to eliminate them.
The meaning of free-form lenses is more than anti-distortion
After the above analysis, you will find that the free-form lens is as magical as it is not advertised. The anti-distortion effect is indistinguishable from the excellent algorithm, and the perspective distortion will not be eliminated, so what is the meaning of it?
The first thing that can be determined is that the suppression of distortion by optical hardware is theoretically more stable than software algorithms. Especially when shooting video, it is difficult for the algorithm to correct the distortion in real time. At this time, the hardware advantage of the free-form surface lens is revealed.
More importantly, it is the lightness of the free-form surface lens, which brings more space to the design of the mobile phone camera module.
By introducing a free-form surface, the structure of the optical system can be made more compact, lighter in weight, and fewer optical components. Therefore, in a similar volume, the application of free-form surfaces can make the space utilization rate of the camera module higher.
For example, let the OPPO Find X3 series and OnePlus 9 Pro series have the opportunity to use the IMX 766 main camera-level outsole sensor to further enhance the image quality.
However, the free-form surface lens has a low processing yield, complex process and expensive equipment, so the cost is relatively high. At present, there are relatively few mobile phones equipped with free-form lens, and only the ultra-wide angle with the most serious distortion problem can be enjoyed.
It can be predicted that when the cost of free-form surface lenses is reduced, whether it is a main lens or a telephoto lens, free-form surface technology may be applied. While suppressing distortion, there is more room for stacking optical systems.
Just like the "dual main camera" and "three main camera" slogans of mobile phone manufacturers now, perhaps we will be able to see mobile phones with "double free-form lens" and even "three free-form lens" in the near future.
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