Making a truly easy-to-use F1.2 lens
Product Leader and Optical Design Lead / Atsuo Kikuchi
―What were some of your goals in designing Sony’s first F1.2 lens?
Kikuchi: We have brought many large-aperture prime lenses to market, but we knew our customers all over the world wanted a faster large-aperture lens. The biggest demand was for a 50mm “standard” F1.2 G Master.
In developing a large-aperture F1.2 lens, we knew we needed to maintain the high levels of resolution and bokeh of the G Master series, while ensuring the lens remained truly easy to use. If we simply prioritised aperture size and the lens ended up too big and heavy, we would lose the compactness and light weight of the lens-body combination that is a principal advantage of the E-mount system. And no matter how excellent its optical properties, a lens that couldn’t extract maximum performance from the outstanding autofocus systems of camera bodies, or couldn’t use autofocus at all, would be a non-starter in terms of customer satisfaction.
In order to achieve the fastest F-number autofocus lens in Alpha history, without compromising autofocus performance and while maintaining ease of handling and portability, we needed to make use of Sony’s most state-of-the-art technology. In addition to its superb optical performance, I think customers who get hold of this lens will be surprised by how light, quick and quiet the autofocus operation is for a large-aperture F1.2 lens.
I think the addition of this F1.2 lens to the Alpha lineup takes the scope of shooting possibilities for creators to another level. It’s a lens that can be used in a wide variety of situations by both professionals and amateurs, from shooting portraits and weddings to landscapes and snaps.
The Alpha lineup already includes the Planar T* FE 50mm F1.4 ZA lens. Comparing the new FE 50mm F1.2 GM lens to that, although the difference between maximum apertures of F1.4 and F1.2 sounds small, it’s actually a full half-stop, with the extra light gathering of the F1.2 lens requiring an effective aperture (diameter) about 17% larger, or an aperture area nearly 40% larger, presenting major design and manufacturing hurdles in realising a compact F1.2 lens.
Overcoming this issue has involved many novel challenges.
One of these was to keep the size of the front lens element down, despite the lens being F1.2, through the adoption of multiple XA (extreme aspherical) lenses, a technology unique to Sony. This avoided the need to increase the front lens size, and allowed us to thoroughly compensate for aberrations that arise in larger diameter lenses.
In order to fully suppress aberrations, an independently-driven floating focus system, comprising two focus groups, was adopted for optimal compensation of aberrations across the entire focus range, including at minimum focus distance.
The focus drive adopts Sony’s proprietary XD (extreme dynamic) Linear Motor, with its combination of high thrust and quietness. Four of these compact direct-drive actuators, with precision control, enable the focus group design to include multiple elements that provide superior aberration compensation.
The result is a lens with G Master level resolution, that extracts the maximum speed, accuracy and tracking performance from the camera’s autofocus, in a lens body only 108mm (4 3/8 in.) long and weighing just 778g (27.5 oz.) — the same as the current Planar lens. We take great pride in having produced an F1.2 lens like none before, that we hope will be of immense value to professionals and enthusiasts alike.
Kikuchi: In order to achieve high optical performance while maintaining a small form factor in an F1.2 lens, Sony’s proprietary XA lenses were adopted, and simulation technologies for resolution, bokeh and chromatic aberration were employed.
Essentially, increasing optical performance is all about how you reduce aberrations.
Historically, 50mm lenses have typically used a Gauss-type layout. The Gauss layout has groups of lens elements distributed symmetrically on either side of a central aperture, which causes aberrations from each side of the aperture to cancel each other out. It is particularly well suited to the 50mm angle of view, so the majority of 50mm lenses in the past have utilised this arrangement.
However, this symmetrical structure by itself only compensates for distortion and curvature of field aberrations, and does not, for example, efficiently compensate for spherical aberration or sagittal flare. In short, this optical design choice wouldn’t have allowed us to achieve the high aberration compensation performance that we were aiming for.
As experienced camera users know, it’s not possible to achieve high resolving power across the entire image without sufficient aberration compensation. Point light sources like stars in the sky should ideally be focused to points wherever they appear in the image, but insufficiently-compensated aberrations may make them appear like fluttering birds or exhibit colour spreading. To counter this, the user may stop down the aperture, but this of course defeats the point of a large-aperture lens.
Our aim with this lens was a level of optical performance where one can be absolutely comfortable shooting at maximum aperture. To achieve that, our optical arrangement partially “breaks” the symmetrical design, and thoroughly suppresses aberrations difficult to suppress with a symmetrical lens design.
Normally, in order to correct for spherical aberration and sagittal flare, symmetrical type lenses tend to have large front elements and may be composed of many elements.
Our new optical arrangement uses just three XA (extreme aspherical) lenses, avoids enlarging the diameter of the front element, and keeps the number of lens elements to a minimum, achieving a compact overall size.
 Extreme aspherical lens (XA lens)
 Contrast (%)  Distance from optical centre of lens (mm) Max. aperture  F8 aperture  Spatial frequency 10 line pairs / mm  30 line pairs / mm Radial values  Tangential values
Kikuchi: F1.2 lenses are known for their rich bokeh, but this lens isn’t just about the amount of bokeh, it’s about delivering an ideal, smooth and creamy bokeh character in keeping with the G Master designation. In particular for portraiture, bokeh plays an extremely important role in making the subject naturally stand out. Bokeh is a very sensual thing, which makes it difficult to engineer, but we knew we needed to nail it to meet customer expectations for an F1.2 G Master.
From the earliest stages of design, we conducted repeated bokeh simulations and adjustments to discern the ideal level of spherical aberration, which let us optimise bokeh and resolution together without compromise on either.
Additionally, during manufacture, the spacing of the elements is adjusted lens by lens for fine control of spherical aberration, mastering the difficult balance between foreground and background bokeh to realise a beautifully neutral overall effect.
I touched earlier on resolution in the manufacture of XA lenses, but the management of surface precision down to submicron levels also suppresses a striping or “onion ring” effect within ball-shaped bokeh.
[1-1] Conventional aspherical lens surface [1-2] Undesirable bokeh result [2-1] XA (extreme aspherical) lens surface [2-2] Beautiful bokeh result
Mechanical Design Lead / Yuichiro Takata
Takata: The soft and beautiful bokeh is also due to the 11-blade circular aperture. The aperture unit was newly developed to maintain an almost circular form even at two stops from fully open.
Since F1.2 is a large aperture, in a conventional design the aperture blades would naturally be large too. And when the aperture is open, the large blades must be moved to an escape space outside the optical path and beyond the effective diameter, which increases the outer diameter of the lens itself. In order to keep the size of the aperture unit down, we had to re-design everything from scratch, from the shape of the blades to the drive mechanism components, one by one.
The aperture unit is extremely important in determining aperture value and exposure. Downsizing its components means higher precision is required in the machining of each, and in the accuracy of the assembly. By thoroughly re-examining the machining and assembly processes, we were able to achieve both miniaturisation and precision.
Takata: The biggest challenge in achieving high-performance autofocus on the F1.2 lens was achieving the extremely high focus accuracy required for shallow depths of field.
Even with a maximum aperture of F1.2, a lens can’t really be said to be “easy to use” unless it delivers commensurate levels of autofocus accuracy and tracking performance. But that’s really very difficult technically. This lens incorporates a variety of technologies and techniques to achieve high-speed, highly accurate autofocus performance even when dealing with the extremely shallow depth of field at F1.2. Four features contribute most significantly: the floating focus structure; the XD Linear Motors; the four focus position sensors; and the optimised balance of the centres of gravity of the two focus lens groups.
The floating focus structure not only improves optical performance; the division of the focus group into two also reduces the weight of each group, helping to achieve a fast and precise autofocus drive.
On the other hand, pinpoint focus accuracy is vital to achieve full resolution performance at F1.2, and this requires precisely synchronised movement of the two focus lens groups that are still relatively large and heavy. This was achieved by Sony’s proprietary XD Linear Motors, which boast high thrust despite their small size.
There is absolutely no room for error with the shallow depth of field at F1.2, so four position sensors are used to track the focus lens groups, to ensure that their exact positions are precisely known at all times.
Finally, in order to deploy the thrust of the XD Linear Motors most efficiently and without wastage, and to make it easier to balance the respective centres of gravity of the two focus groups, a fixed optical group is inserted between the two focus groups. This aligns the thrust point of the motors with the centre of gravity of each focus group, maximising the efficiency of power transmission and eliminating wasteful thrust, and further contributing to the achievement of a high-speed, high-accuracy and quiet autofocus drive.
Actuator Control Lead / Yuki Mizuno
Mizuno: Allow me to add some more information about the focus drive.
First of all, this lens uses a total of four direct-drive XD Linear Motors, with two motors assigned to each of the two focus lens groups.
Each motor was designed based on data from Sony’s proprietary motor design simulation. Advances in motor design simulation technology have made it possible to develop highly efficient motors, producing sufficient power despite severe size constraints, and achieving high reliability in a variety of harsh environments. The ability to design motors with a specification and size optimally suited to this lens contributed to compactness without compromise on performance.
Normally, rotary-type actuators are used to drive heavy focus groups, but the cams and gears that convert rotary into linear motion inevitably cause power loss. And having many mechanical parts involved can create noise and vibration.
This wouldn’t do for the high-performance F1.2 lens that we were aiming for, so we decided to use small but powerful motors that can drive the focus groups directly and linearly, adopting XD Linear Motors that feature high speed, with low noise and vibration.
However, because linear-type motors don’t have a speed reduction mechanism, in order to realise high-speed and high-accuracy autofocus, extremely responsive control is required.
Specifically, the four sensors that I mentioned earlier detect precisely the positions of the focus groups, and provide that positional data to the control system in an ultra-fast feedback cycle that boosts responsiveness. This also utilises Sony’s proprietary control simulation technology. Many patterns of lens motion and stopping were thoroughly and repeatedly simulated, tested on actual hardware, and analyzed. Finally tuning was performed to give a smooth actuator motion, optimal for this lens, from acceleration to braking.
This fine control reduces drive noise and vibration to the extent that one might question whether the lens is even moving. The XD Linear Motors are software-controlled to deliver maximum autofocus speed and responsiveness, allowing us to create a compact lens boasting superb optical performance.
Kikuchi: I’d also like to touch on how this F1.2 lens makes full use of the camera body’s functionality. Sony develops all of the essential components from the device level, including the image sensor, so cameras and lenses are developed in-house simultaneously as a total system. When we develop interchangeable lenses, we also anticipate future advances in bodies, to ensure that lenses will be able to extract maximum performance from future bodies.
Naturally this lens is ideal for use with the new α1, announced in January 2021, with 30fps continuous shooting, 8K, and 4K120p high-resolution movie shooting. But we have also tried to anticipate future trends in camera bodies. Our aim is to pursue designs that will provide maximum performance not only now but also going forward.
Takata: We’ve developed this lens with no compromise also on operability, so that it can be used in professional situations.
For example, despite its compact casing, customisable focus hold buttons are provided on both the top and the side of the lens, giving the same operating feel whether shooting in horizontal position or shooting portraiture in vertical position.
Mizuno: We also designed the F1.2 with manual focusing in mind, paying special attention to the positioning of the focus ring, its torque, and feel when being rotated. The lens is equipped with Linear Response MF, which responds directly and linearly to focus ring rotation to ensure precise focus adjustment, even reacting to very small movements of the focus ring. The demand for positional accuracy is severe at F1.2, but we developed the lens to meet that demand.