Development concept: design a really easy to use f1.2 lens
Product and optical design director: atsuo Kikuchi
Q: What goals did you set when developing Sony’s first f1.2 lens?
Judi: Sony has released many large aperture fixed focus lenses, but we know that users all over the world want to have a higher speed and brighter large aperture lens, and the biggest demand is the 50mm “standard” (focal length) f1.2 large aperture g master lens.
In the development of f1.2 large aperture lens, we should not only maintain the high resolution and astigmatism of G master lens series, but also consider the ease of use of the lens. If we only consider the size of the aperture, and the lens eventually becomes large and heavy, we will lose the compactness and lightness of the lens and fuselage combination, which means giving up the main advantage of e-bayonet system. And no matter how excellent the optical performance of the lens, if you can not achieve excellent auto focus, or simply can not achieve auto focus, I believe many users will not be satisfied.
In order to design an autofocus lens with the largest aperture in alpha history and maintain professional control and portability without compromising the autofocus performance, we have adopted Sony’s most advanced technology. In addition to the excellent optical performance, I believe users will also be satisfied with the compact and lightweight design of the large aperture f1.2 lens, as well as the quiet, fast and accurate autofocus performance.
I think the creation possibility of users will be further improved after adding f1.2 lens to alpha series products. It is a lens that professional photographers and photographers can use in all kinds of shooting subjects, such as portrait, wedding, scenery and street shooting.
Alpha series products already include planar t * Fe 50mm f1.4 Za lens. Compared with the new Fe 50mm f1.2 GM lens, although the difference between the maximum aperture of f1.4 and f1.2 sounds very small, it is actually a half aperture gap. The effective aperture (diameter) of f1.2 lens needs to be about 17% larger or the aperture area is nearly 40% larger for additional focus. This is when designing a miniaturized f1.2 large aperture lens, Main design and manufacturing barriers.
Overcoming this problem involves many new challenges. One of them is to use multiple Xa (super aspheric) lenses – Sony’s unique technology, which not only reduces the size of the front lens, but also enables the lens to achieve f1.2 large aperture, and effectively compensates for the aberrations in large diameter lenses.
In order to suppress the aberration effectively, a floating focusing system with two focusing lens groups is adopted to compensate the aberration in the whole focusing range (including the nearest focusing distance).
Focusing drive adopts Sony’s unique XD linear motor, which has the characteristics of high driving force and quiet. The four compact direct drive linear motors have precise control function, so the design of multiple focus lens groups can effectively compensate the aberration.
Finally, a compact and lightweight design with a lens length of only 108mm and a weight of only 778g is realized, which has the iconic resolution of master g lens, and can be combined with Sony micro single ™ Achieve fast, accurate and quiet auto focus and tracking performance. We are proud to design such an excellent f1.2 lens and hope to help professional photographers and enthusiasts to shoot more excellent works.
Uncompromising optical design
High resolution at f1.2 aperture
Judi: in order to keep a small size in the f1.2 lens and achieve high optical performance, the f1.2 GM adopts Sony’s unique XA lens and adopts resolution, astigmatism and chromatic aberration simulation technology.
In essence, improving optical performance is related to reducing aberration.
Historically, a 50 mm lens usually has a Gaussian structure. The Gaussian structure has a group of lenses, which are symmetrically distributed on both sides of the central aperture, so that the aberrations on both sides of the aperture can cancel each other. Gaussian structure is especially suitable for 50 mm viewing angle, so most 50 mm lenses used this structure in the past.
However, the symmetrical structure itself only compensates the distortion and curvature of the image field, and does not effectively compensate the spherical aberration or sagittal flare. In short, this optical design will not give us the desired high aberration compensation performance.
As experienced camera users know, it is impossible to achieve high resolution in the whole image range without enough aberration compensation. Ideally, a point light source (such as a star in the sky) should be focused on the image as a point, but under compensated aberrations may make them look like flying birds or color diffused spots. In order to solve this problem, users can reduce the aperture, but this will make the large aperture lens unable to play its best advantage.
We hope that the optical performance of this lens can meet the harsh needs of users shooting under the maximum aperture. For this reason, we break the symmetry design in the optical structure, and effectively suppress the aberrations which are difficult to control in the symmetrical lens design.
In order to correct spherical aberrations and sagittal flares, usually symmetrical lenses tend to use larger front lenses, which are composed of many lenses. Our new optical design uses only three Xa (super aspheric) lenses, which avoids enlarging the diameter of the front lens and reduces the number of lenses, thus achieving an overall compact size.
As the name suggests, the “aspheric” surface curvature of XA lens is not constant, but changes from the center to the edge. The three XA lenses used in the lens all use Sony’s unique optical simulation technology, and the shape of the lens is optimized after many iterations.
The surface accuracy of XA lens used in G master lens series has been as low as submicron level. The f1.2 large aperture and large external diameter lenses of the lens require that the accuracy of each step in the manufacturing process of the three XA lenses should be substantially improved to achieve the required higher surface accuracy, which is the highest level manufacturing challenge we have ever faced. But through the integrated design, and improved the manufacturing process of each step, facing the challenge of new technology, we have realized the large diameter and high-precision XA lens.
Let’s take a look at the lens structure diagram (above). The second XA lens in the front row helps to reduce the number of lenses required by the front group to a great extent, and also helps to reduce its size and weight. Using large-diameter aspheric lens in this position, the manufacturing accuracy can only be achieved by us at present. This huge advantage supports the whole optical design of compact f1.2 lens.
Sony’s unique chromatic aberration simulation technology is used to optimize the combination of glass materials, effectively reducing chromatic aberration and dispersion. Despite the challenge of large aperture, it still achieves a high level of resolution and contrast, which is consistent with the quality of G master lens series.
When optical engineers look at the lens structure, they sometimes think that “this design does not have much effect on aberration correction”（ (laughter) as an engineer, my goal is to achieve the most effective aberration correction with the least number of lenses. In other words, it is to seek a compact solution that can provide the overall lens while maintaining the optical performance. It can be seen from the structure diagram of Fe 50 mm f1.2 GM above that there is no waste or compromise in this design, and the curvature of all lenses is repeatedly considered due to its influence on aberration. I hope users can experience and enjoy the combination of compactness and optical performance brought by the final optical design.
F1.2 lens brings smooth and rich scatter
Judi: f1.2 lens is famous for its rich and soft scenery. This kind of lens not only has rich scenery, but also has smooth and creamy scenery, which achieves the symbolic background effect of master g lens. Especially for portrait photography, soft scenery can make the shooting object stand out naturally, which is very important for taking beautiful background virtual effect photos. Scatter is very important, it makes lens design difficult, but we need to improve it to meet the user’s expectation of f1.2 large aperture g master lens.
From the initial stage of the design, we have repeatedly simulated and adjusted the scatter to identify the ideal spherical aberration level, which enables us to optimize the scatter and resolution together without affecting the two.
In addition, in the manufacturing process, the lens spacing should be adjusted one by one to accurately control the spherical aberration – to grasp the complex balance between the foreground and background astigmatism, so as to achieve the aesthetic overall effect.
Xa lens can not only meet the requirements of high resolution and control the surface accuracy to submicron level, but also suppress the stripe and “onion ring” effect in spherical astigmatism.
[1-1] traditional aspheric lens surface [1-2] poor astigmatism effect [2-1] Xa (Advanced aspheric) lens surface [2-2] soft astigmatism effect
Mechanical design director: yuichiro Takata
Takata: 11 circular aperture blades also make soft and beautiful scenery possible. The lens adopts the newly developed aperture unit, which can keep almost circular shape even if it is shrunk by two steps.
Because f1.2 is a large aperture, so in the conventional design, the aperture blade will be large. And when the aperture is open, the large blade must move to the outside of the light path and exceed the effective diameter of the escape space, which will increase the outer diameter of the lens itself. In order to reduce the size of the aperture unit, we have to redesign everything from the shape of the blade to the drive mechanism components.
Aperture unit is very important for determining aperture value and exposure. Reducing the size of components means that higher precision is needed in the processing and assembly accuracy of each component. By checking the machining and assembly process, miniaturization and high precision are realized.
The circular aperture can be maintained even if two gears are retracted
Linear direct drive motor: the key to miniaturization
Takata: in order to achieve high optical performance and autofocus, the mechanical and software control teams need to work closely together.
As I explained earlier, maintaining high performance over the entire focus range requires two focus groups consisting of multiple elements. The large diameter of the f1.2 lens will inevitably increase the weight of the focusing lens group. Increasing the weight of the focusing lens group will increase the image focusing speed, as well as the noise and vibration of the focusing motor.
The problem is how to maintain excellent resolution and astigmatism without sacrificing autofocus speed. The solution is to use Sony’s own direct drive XD linear motor for focusing.
Auto focus and mechanical design
F1.2 aperture for fast and accurate auto focus
Takata: to achieve high-performance autofocus on the f1.2 lens, the biggest challenge is to achieve the high focusing accuracy required for shallow depth of field.
Even if the maximum aperture is f1.2, it can not be said to be an “easy-to-use” lens, unless it provides excellent auto focusing accuracy and tracking focusing performance, but it is difficult to achieve technically. The lens combines a variety of new technologies to achieve fast and high-precision autofocus even in the extremely shallow depth of field of f1.2. Four parts contribute the most: floating focusing structure; XD linear motor; Four focusing position sensors; And the balance optimization of the center of gravity of the two focus lens groups.
The floating focusing structure not only improves the optical performance, but also reduces the weight of each group by dividing the focusing lens into two parts, which helps to achieve fast and accurate auto focusing.
On the other hand, accurate alignment is very important for the resolution performance of f1.2 full open. Sony’s unique XD linear motor drives the large and heavy focus lens groups to move synchronously, so as to achieve fast and accurate focusing. Although XD linear motor is small, its driving force is very high.
The shallow depth of field of f1.2 has no margin of error when shooting, so four position sensors are used to track the focus lens group to ensure that its accurate position is always known accurately.
Finally, in order to effectively use the thrust of XD linear motor without loss, and make the center of gravity of the two focus lens groups more easily balanced, a fixed optical component is inserted between the two focus lens groups. This makes the thrust point of the motor align with the center of gravity of each focusing lens group, so as to maximize the power transmission efficiency and eliminate the loss of thrust, which is conducive to further realize fast, accurate and quiet auto focusing.
Head of drive control: Yuki MIZUNO
Mizuno: please allow me to add more information about the focusing motor.
Firstly, four direct drive XD linear motors are used in the lens, and two motors are assigned to the two focus lens groups.
Each motor is designed according to the unique motor design simulation data of Sony. With the development of motor design simulation technology, it is possible to develop high efficiency motor. Although the size is strictly limited, it can still produce enough power and has very high reliability in all kinds of harsh environments. Design the motor with the most suitable specifications and size for the lens to help achieve compactness without affecting performance.
Generally speaking, the rotary motor is used to drive the heavy focus lens group, but the cam and gear which convert the rotation into linear motion will inevitably lead to power loss, and many mechanical parts will produce noise and vibration.
For high-performance f1.2 lens, this is not feasible, so we decided to use a small but powerful motor to drive the focus lens group directly and linearly, and XD linear motor to achieve fast, quiet and low vibration.
But because the linear motor has no deceleration mechanism, in order to achieve fast and accurate auto focus, it needs very sensitive control.
Specifically, the four sensors I mentioned above accurately detect the position of the focusing lens group, and provide the position data to the control system with a fast feedback cycle, thus improving the response speed, which also makes use of Sony’s unique control simulation technology. Many modes of lens movement and stop have been simulated repeatedly, and tested and analyzed on actual hardware. Finally, adjustments were made to make the driver move smoothly from acceleration to rest – the best choice for the lens.
This fine control reduces the driving noise and vibration to such a low level that the user may doubt whether the lens is moving. XD direct drive linear motor is controlled by software, which can provide maximum autofocus and response speed, enabling us to produce compact lens with excellent optical performance.
Lenses that meet Sony’s current and future fuselage needs
Judi: I also want to talk about how f1.2 lens can make full use of the performance of camera body. Sony has developed all the necessary components from the system level, including the image sensor, so the camera and lens are developed internally as a whole system. When we develop the replaceable lens, we should also take into account the future development of the fuselage, so that the lens can get the best performance from the future fuselage.
Of course, this lens is very suitable for use with the new alpha 1 released in January 2021. It can be used for 30fps continuous shooting, 8K and 4K 120P high-resolution video shooting. We also try to predict the future trend of fuselage. Our goal is not only to meet the needs of the current fuselage, but also to meet the needs of future models with high image quality and high performance.
Reliability and ease of use
Takada: the lens we developed is also uncompromising in operability, so it can meet the needs of professional photographers.
For example, despite its compact appearance, it has customizable focus hold buttons on the top and side of the lens, which provides the same professional experience for shooting portrait in both horizontal and vertical positions.
Mizuno: when we design Fe 50 mm f1.2 GM, we also consider the manual focusing, especially the position of the focusing ring, and pay attention to the torque and the hand feel when rotating. The lens is equipped with a linear response manual focusing system, which allows the focusing ring to respond directly and linearly, so as to ensure accurate focusing adjustment, and even respond to the small movement of the focusing ring. In f1.2, the requirement of position accuracy is very high, but we have developed a lens to meet the requirement.
Strong and durable, can be used in complex and harsh environment
Judi: the lens is well sealed to prevent dirt, dust and water splashing – the dust and moisture-proof design provides guarantee for users.
The fluoride coating of the front lens can reduce the possibility of water stains, oil stains, dust and other pollutants on the lens surface.
Mizuno: we also take into account changes in ambient temperature. For example, the properties of mechanical and electrical components (such as the thrust of the focusing motor) change with changes in environment and temperature. The software of the lens can continuously optimize the performance by independently calculating various control parameters, and can maintain the accuracy even under harsh conditions.
Finally, even when shooting in harsh outdoor conditions (such as extremely cold or hot environment), users can use this high-performance lens to shoot safely.
F1.2 lens worth having
Judi: as an optical designer, I can say without exaggeration that this lens is the pinnacle of G master lens series, with high resolution and excellent scatter. I hope users can experience the soft background effect and high resolution brought by this f1.2 lens.
Although the maximum aperture is f1.2, the lens has achieved a balance between compactness and high performance, and has a lot of professional performance. Lens contains many new technologies of Sony, so as a lens engineer, I’d like to see users use it to shoot different subjects.
Shuiye: This is a lens that can be used for a variety of shooting themes, meeting the different shooting needs of professional photographers and enthusiasts. It is not only suitable for shooting portraits and weddings, but also has high-performance autofocus function, which enables it to capture fleeting moments, such as tracking fast-moving objects in sports events.
Takada: This compact Fe 50 mm f1.2 GM lens also provides excellent performance for video capture. When shooting with hand-held or stabilizer, the auto focus performance can easily track the subject even when the depth of field of f1.2 is shallow. The combination of excellent auto focusing, quiet and smooth aperture ring, and accurate and responsive manual focusing ring makes photographers very popular. I hope users will like this new form of video visual expression.
By providing a new shooting experience, the lens fully embodies the value and potential of G master lens series.
Author: Shen Pei