Sony engineers discuss the concept of the G Master lens, born out of technological innovation and uncompromising optical design.
Sony engineers discuss the concept of the G Master lens, born out of technological innovation and uncompromising optical design.
Back in 2013 when we launched the α7 series, we assembled a lineup prioritising F4 zoom lenses and relatively small prime lenses based on the concept of small size and high performance. Against this backdrop, we heard from a lot of customers demanding F2.8 zoom lenses and F1.4 prime lenses.
Meanwhile, the camera bodies had already evolved from the α7R, with a resolution of about 36.4 million pixels, to the α7R II, which had about 42.4 million pixels and was equipped with 5-axis image stabilisation.
Image sensor technology continues to advance, and we believe high resolutions are a universal value that will become increasingly important. Lens bokeh is another universal value, and both will continue to be in demand. In this spirit, with the development of our large-aperture F2.8 zoom and F1.4 prime lenses, we've launched a new brand that’s focused on effectively combining high resolution and bokeh.
Sony knows the future of image sensors, so this lens will remain relevant in the face of continually evolving camera performance
The three products we have developed under the new G Master brand are: the FE 24–70mm F2.8 GM standard zoom lens, which targets prime lens class resolution performance at any focal length, the FE 85mm F1.4 GM portrait lens, which places a premium on bokeh rendering from the sharpest focus to the background, and the FE 70–200mm F2.8 GM OSS telephoto zoom lens, which is devoted to capturing powerful resolution in the telephoto end and bokeh in the portrait spectrum.
These lenses employ a variety of innovative technologies, including our newly developed XA (extreme aspherical) lens elements. Sony is a leading manufacturer of image sensors and has a clear vision of how future image sensors will evolve. The duty of the G Master series, I believe, is to anticipate this progress and deliver lenses that will remain relevant well into the future.
Deputy General Manager - Lens Mechanical Design Department
In developing the G Master, we adopted an XA (extreme aspherical) lens element with a surface accuracy of 0.01 microns. To begin with, aspherical lenses are very difficult to manufacture, and it was a challenge for us to see how far we could raise the surface accuracy.
In fact, at the time we were developing this technology, we collaborated with the engineers at a production site belonging to Sony Global Manufacturing & Operations Corporation in Aichi Prefecture, Japan, where they have aspherical lens production technology, and after reviewing every single manufacturing process from scratch, we introduced a new process.
I'm confident that the XA lenses we were finally able to produce are of the highest quality in the world. I think the reason we were able to perfect them to such a high degree was because we were able to use Sony's unique proprietary technology and develop it in close collaboration with the production facility.
Usually, in lens design, we don't give all lens elements the same effectiveness and sensitivity; rather, we optimally adjust the design by assigning roles to each lens element. Similarly, for the G Master, because we have an XA lens element with high surface accuracy, we can provide features such as sensitivity and aberration compensation capability. As a result, we can make the entire lens deliver higher resolution than ever before.
I think the reason the G Master is able to achieve both resolutions from the centre to the corners of the image and very natural bokeh is precisely because we were able to create XA lens elements with extremely high 0.01-micron surface accuracy. It's not an exaggeration to say that the G Master would not have been possible without the development of our XA lens elements.
Creating a more effective combination of conflicting elements with a frequency of 50 line-pairs per millimeter as an MTF baseline
Since high resolution and beautiful bokeh are inherently at odds, it's very difficult to bring them together into their ultimate form. With respect to resolution, we adopted an MTF1 with a spatial frequency of 50 line-pairs per millimeter as our design baseline, the most challenging standard ever.
Higher spatial frequency means the lens can better reproduce subtle changes in contrast. In order to achieve that target, we completely removed multiple optical aberrations such as chromatic and coma aberrations at the design stage. Specifically, to eliminate aberrations at the corners of the image, we repeated numerous simulations, removing aberrations bit by bit until we had the final optical design.
Of course, merely achieving higher spatial frequency does not guarantee a good lens, since there may be many low-frequency components depending on the subject to be captured. Ultimately, the goal should be a photo that looks good when you see it, so what matters is achieving a design that's well balanced from the low frequencies to the high frequencies.
At the lens design stage, as we're increasing the resolution, we have to continually check on the bokeh. To do this, we used bokeh simulation to make sure the bokeh was not being adversely affected as the MTF performance improved.
That said, lens bokeh is a highly complex subject, and there are various types. For example, front and rear, and from close range to long range. Since the type of bokeh also varies depending on the focal length, it's not something that can be judged just by simulation.
The bokeh simulations must therefore be skillfully integrated, using Sony's accumulated experience and know-how. In this way, we continually seek out bokeh effects that our customers will find beautiful.
Within optical design, there are several elements that have an impact on bokeh, but it's difficult to eliminate them completely at the optical design stage. There are bound to be individual differences in the final product, so G Master series lenses are checked one-by-one during the manufacturing process and adjusted to ensure that they produce ideal bokeh before shipment.
To perform these adjustments, you need to do things such as build new adjustment equipment and include an adjustment mechanism in the lens itself. The mechanisms to implement this processing step are required ahead of time. However, by adjusting the bokeh like that, we can deliver top-notch lenses to our customers, and I think this kind of attitude and spirit is an important part of the G Master mentality.
The FE 85mm F1.4 GM and FE 70–200mm F2.8 GM OSS uses an 11-blade circular aperture for the first time in an α lens. The development of the current 11-blade aperture was inspired by the need to keep the bokeh shapes circular to the greatest extent possible at large-aperture settings. A further benefit of increasing the number of blades is that it allows a more compact design for aperture units with very large aperture diameters.
Lenses are adjusted one-by-one during the manufacturing process to ensure that they produce the ideal bokeh
Bokeh with 11-blade aperture
Bokeh with 7-blade aperture
Resolution and bokeh are optimised by the optical design, then put to the test by Sony's actuator technology
Even if you establish a balance between high resolution and bokeh and reach the highest levels of performance, you won't get a chance to experience that performance without equal focusing accuracy. For that reason, top-notch accuracy was required there as well. In order to get the most out of the lens performance, while the optical designers were enhancing it as far as it would go, we gave the G Master incredible focusing capabilities by combining Sony's innovative actuators, developed using our proprietary technology, with mechanisms optimised to the optical characteristics of each lens.
The FE 70–200mm F2.8 GM OSS deals with the weight of the focus lens by splitting the focus group, which is usually one piece, into two separate focus groups. This distributes the weight. Furthermore, by combining the focus groups with independent actuators and controlling them independently, the lens achieves smoother focusing not only for still images, but for video as well. Splitting the focus group in two also creates a floating structure that makes it possible to effectively correct various aberrations at all subject distances, and thanks to the floating structure, which is capable of fully independent control, you can adjust the position of each lens element one by one to its ideal position in a way that's impossible in a typical floating structure, making it possible to get the most out of the inherent performance of the lenses.
1. Ring drive SSM 2. Linear motor
Innovative actuator technology enables an entirely different shooting style
In addition, the FE 85mm F1.4 GM and FE 70–200mm F2.8 GM OSS are equipped with new-generation ring drive SSM motor systems that optimises the ring drive SSMs used in A-mount lenses for the contrast AF in mirrorless interchangeable-lens cameras. Mirrorless interchangeable-lens cameras that also use contrast AF require a small reciprocating drive when focusing, which means smooth rotation is necessary.
The ring drive SSM is given a smoother rotation by combining it, for the first time, with ball bearings. The FE 85mm F1.4 GM is also equipped with two position sensors that directly detect and control the focus lens. The excellent ring drive SSM, dual position detection sensors and an advanced drive algorithm make it possible to control the position of the lens at the 1-micron level, enabling extremely high-precision AF.
I'm really hoping people will try out the "Eye AF (autofocus)" feature, which brings the subject's eyes into focus, in a combination of the FE 85mm F1.4 GM and the α7 series, because once they do, I think they'll really realise the high focusing accuracy of this lens. Traditional 85mm F1.4 lenses, due to the shallowness of their depth of field, have sometimes had trouble bringing the eyes completely into focus with AF.
Many professionals use manual focus for precise focusing of 85mm F1.4 lenses, so I think it's wonderful that we were able to make it easier to reach that level of accuracy with AF. Despite the fact that the 85mm F1.4 is a lens with a very shallow depth of field, we've achieved enough accuracy to drive focus lenses to the point where photographers can focus on not just the pupil, but the eyelids, the ends of the eyelashes, the surface of the pupil, or the reflection, according to their intention.
The innovative actuator technology built into these lenses is what enabled us to achieve this degree of accuracy. I think this will lead to a completely different shooting style, since before photographers had to concentrate on precise focusing, whereas now they are free to change the angle of view and let the subject move around, all the while keeping the pupils in focus.
A message from Akira Shiraishi
I have a particular fondness for the FE 85 mm F1.4 GM. Ten years ago, I was interviewed about the 85mm F1.4 A-mount lens: among the α lenses, the 85mm F1.4 is something iconic, with exceptional specs. I think this is an excellent lens to represent the α brand.
In creating this new 85mm F1.4, I wanted to make it the ultimate lens to surpass all the lenses that came before it. I think that's exactly what we’ve accomplished, and I hope our feelings about the FE 85mm F1.4 GM will be shared by others. And by seriously listening to the feedback from our customers on the F4 series, we were able to improve the performance of our two other lenses in the G Master series, the FE 24-70mm F2.8 GM and the FE 70-200mm F2.8 GM OSS. I hope customers currently using the F4 series will give these new F2.8 models a try.
A message from Masanori Kishi
Leading up to the simultaneous release of these three exceptional lenses under the new G Master brand, all of us developers continuously raised the bar by packing as much as we could into each lens, including fine-grained operability, without any compromises. Since the launch of the α7 series, we've received lots of valuable opinions and information from a number of professional users.
We brought these products to market by working out every detail to its logical conclusion, whether in regard to operability, performance or other features, in a way that responds to the input from these users. I believe we've created lenses I can truly recommend with confidence, so it will make me happy to see people actually use these lenses and have fun taking pictures with them. I hope people will enjoy the unprecedented fusion of high resolution and beautiful bokeh that this new G Master series delivers.
High resolution and spectacular bokeh come together in lenses that will continue to deliver ultimate performance with the most advanced camera bodies for many years to come.