G MASTER

FE 85 mm F1.4 GM 

Engineers’ Commentary

The development team discusses the design philosophy and appeal of the FE 85 mm F1.4 GM.

Concept

Development Concept

A lens with uncompromising optical performance to epitomize the G Master series

Maenishi: The 85 mm prime, which is the most popular portrait lens, is a highly symbolic lens, so companies everywhere are putting high-end models on the market. In developing this new model, we also wanted to beat our rivals and create something that would mark a new era in lenses. When it comes to portrait lenses in particular, we were fixated on the background bokeh. However, bokeh-oriented lenses tend to depict a soft, floaty feeling. The SEL85F14GM accommodates both high resolution and beautiful bokeh without any compromises. This is precisely the concept behind the G Master series, but being a prime lens, this one's aimed at an even higher level.

As a result, we decided in the early stages of the project to produce a lens that would make no compromise whatsoever in optical performance. The design of a lens always requires a balance between the optical design and the mechanical design. That's because it's necessary to consider not only optical performance but a variety of mechanical elements, such as size, focusing speed, and accuracy. For this lens, the biggest challenge was how to integrate the various mechanical elements without compromising the highest level of optical design. As a result, many parts of the mechanical design gave us great difficulty, but in the end, I think we were able to achieve the ideal state of optical performance.

Mechanical design Junichi Maenishi

Inheriting the beautiful bokeh from existing α lenses while offering higher resolution for the next generation of cameras

Maruyama: The α series includes a line of 85 mm lenses that have received high praise since the Minolta era. Most recently, the A-mount SEL85F14Z has earned a reputation for beautiful bokeh, but the aim this time was to further boost reproduction performance. The goal was to enhance the beauty of the bokeh inherited from existing α lenses while achieving the high resolution required for the next generation of cameras, because when the softness of the bokeh is accompanied by high resolving power on the focal plane, it leads to an overwhelming sense of depth. However, because there is an inherent contradiction between the beauty of the bokeh and the resolving power of the lens, the goal was to maximize both of them in a perfect balance. We lavished the SEL85F14GM with Sony's advanced technologies, starting with its XA (extreme aspherical) lens element and nano-AR coating, so that the overall optical system, including the actuators, is optimal for shooting portraits.

Optical Design Masaki Maruyama

Optical Design

A passion for optical design

people

Taking advantage of tight resolution and beautiful bokeh even at the maximum aperture 

Maruyama: In today's world, where resolving power is emphasized, many lenses offer bokeh that is too hard, or that is soft but has low resolution. The history of lens design demonstrates that it is very difficult to achieve a balance between high resolution and beautiful bokeh. We therefore first determined the aberration to aim for in order to achieve an ideal coexistence of resolving power and bokeh. As we proceeded to design for this goal, we reached the conclusion that an aspherical lens element would be essential. However, the use of aspherical lenses is known to cause a phenomenon called "onion-ring bokeh," whereby microscopic irregularities on the aspherical lens surface are transferred to the bokeh, which runs counter to our design philosophy of wanting to beautify the bokeh. We solved this problem with our "XA (extreme aspherical)" lens element. By achieving high surface accuracy at the 0.01-micron level, we succeeded in minimizing the onion-ring bokeh that tends to occur with aspherical lenses. This made it possible to deliver the optical performance we were aiming for without compromising the bokeh.

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The SEL85F14GM also uses three ED (Extra-low Dispersion) glass elements to significantly reduce chromatic aberration and achieve high reproduction performance. Until now, it’s been impossible to avoid the outbreaks of purple fringing caused by chromatic aberrations near the maximum aperture of large-aperture mid-range telephoto lenses. For this lens, however, we put a lot of effort into reproduction, even at wide-open apertures, to bring this purple fringing firmly under control so that the customer can make full use of maximum aperture. You can immediately see this for yourself if you take a photo of a person dressed in black and white stripes, for example. I think there's a common impression that with 85 mm F1.4 lenses, there's a lack of extreme sharpness near the widest aperture, so it's better to stop down a little, but the lens performance is now at the point where you can actively use it for good resolution, even with the aperture wide open. Since the softness and beauty of the bokeh are most prominent at maximum aperture, this lets you take full advantage of the expressive power of the F1.4 lens in your photography.

opticaldesign

In addition, as you can see by looking at the MTF (Modulation Transfer Function) curves, the SEL85F14GM maintains very high optical performance at the edges. In other words, the picture will look good no matter where in the image the subject was captured. In addition, because it achieves clear reproduction with high contrast at any aperture, even when capturing a landscape at a small aperture like F4 or F5.6 in combination with a high-resolution camera such as the α7R II, when you zoom to the actual pixel size, there's a tremendous sense of depth to the image. I’d like everyone to give it a try.

camera

Truly a lens that was born to take portraits

Maruyama: With respect to bokeh, we set targets that were even higher than the highly regarded bokeh of the SEL85F14Z. We had thorough discussions about our optical performance goals, with lens designers from the Minolta era and veteran designers who understood the design philosophy of the 85 mm. On top of that, we proceeded with its development by utilizing the bokeh simulator we introduced on this occasion to verify the qualities of the bokeh at the design stage. In addition to this, in designing the bokeh to be softer, attention was paid not just to aberrations but also to the impact on vignetting. Incidentally, the aberrations in the SEL85F14GM are individually adjusted one by one in the manufacturing process to achieve the ideal bokeh, but I think evaluating the beauty of bokeh also involves elements of subjective feeling and so also depends on the customer's preference. We therefore sought the cooperation of quality assurance personnel familiar with the Minolta-era 85 mm lenses in order to determine the target values for the final individual adjustments, while comparing the results to the bokeh from the 85 mm lenses of the past.

Another thing contributing to the realization of beautiful bokeh is the 11-blade circular aperture, which has been adopted for the first time in an α lens. The most noticeable difference with the change from 9 to 11 blades is the appearance of the bokeh at smaller apertures. At the maximum aperture, the bokeh makes the same perfect circle as the 9-blade version, but in the 11-blade version, the expression is even closer to a perfect circle, even when the aperture is narrowed to about F2.8. Since the aperture setting one or two stops down from full open is often used in portrait photography, you can get a beautiful image with circular bokeh even when you want to increase the depth of field a little while leaving the background blurred. I think that now more than ever, it's possible to meet the most delicate expressive intentions of the photographer. You could say that the SEL85F14GM, which includes these features, is truly a lens that was born to take portraits.

Mechanical Design

A passion for mechanical design

Micron-order control

Micron-order control using a new-generation ring drive SSM

Maenishi: As a result of prioritizing optical design and increasing the resolution to the limit, the focus lens became larger and heavier. In addition, because of the extremely shallow depth of field at F1.4, the focus lens requires micron-order stopping accuracy. The biggest challenge for the mechanical design was how to move the larger, heavier focus lens at high speed and stop it with a high degree of accuracy. The key component was the ring drive SSM, which was adopted for the first time in the focus of an E-mount lens. In order to take full advantage of high-precision contrast AF, the powerful ring drive SSM was placed under the control of two position detection sensors and optimized for the E-mount lens format. A typical ring drive SSM controls the position of the focus lens by the amount of rotation of the focus drive ring, but this lens, in addition to that, is also equipped with a linear motion position sensor to directly detect the position of the focus lens itself. Since control is based on two pieces of information, very high stopping accuracy and fine AF control are possible.

Another thing contributing to improved accuracy is the ball-bearing structure joined to the ring SSM. This ball-bearing structure enables a smooth focus drive by smoothly receiving the force with which the cam ring is pushed in the direction of the optical axis. This helps eliminate mechanical backlash and increase accuracy. While "ring drive SSM" may sound like a technology from the past, the combination of ball bearings and control by two position sensors yields a new-generation ring drive SSM with both power and precision.

Focus

Perfect autofocus at a shallow depth of field of aperture F1.4

Maenishi: On the FE 85 mm F1.4 GM, where the focus optics alone exceed 100 g, performing adjustments at the micron level requires more than just an ingenious mechanical design, so we also developed a new control system. Because the mechanical design and control team shared a common goal, namely "perfect autofocus at a shallow depth of field of aperture F1.4," we spent a lot of time repeating countless test shots while varying the situation and shooting conditions in order to push the accuracy from both the mechanical and control sides. It was worth the trouble, because we ended up with a lens that can comfortably focus even when using the Eye AF function of the α7R II and other compatible cameras.

Furthermore, I think that portrait photography on the 85 mm F1.4 is often shot using manual focus to obtain an exact focus, and that feeling of manipulating the focus manually is associated with a very-high level of professionalism. We implemented fine-tuning by paying attention to that feeling of direct manipulation you get when you turn the focusing ring to drive the focus lens in accordance with the desired image. When it comes to shallow focus lenses, I think it's been difficult up to now to adjust the focus exactly using the manual focus on an optical viewfinder, and the image slips slightly out of focus when you magnify it. But with an EVF (electronic viewfinder), you can easily zoom during shooting, and it's easy to force an exact focus manually. We optimized the drive controls of the actuators to be able to achieve an operational feeling similar to manual focus, while taking advantage of these benefits of the EVF, but without all the stress of manual focus. The resolution at maximum aperture is much higher thanks to the no-compromise optical design, so whether you shoot in auto or manual focus, I think you'll enjoy comfortable focusing at a level of detail that was not possible until now.

 

finally

Finally

Maenishi: There are many resolution-oriented or bokeh-oriented lenses to choose from, but if you want to obtain a superior expressive power that combines them both, I think this lens will be your top choice. This lens was created by paying single-minded attention to operability and functionality, such as autofocus and manual focus, and is even equipped with a focus hold button, which is rare in a lens of this class. This button can even be assigned functions other than focus hold, for example the Eye AF function, so I hope customers will enjoy playing with these function assignments. It's quite different from the lenses that have come before it, so I hope people will pick it up and give it a try.

Maruyama: This lens delivers an optical performance worthy of the G Master series, not only in terms of its bokeh and resolution, but in every respect, including suppression of chromatic aberrations and distortion aberrations. I also recommend it for photography at maximum aperture, but if you’re stopping down the aperture to F1.7, F2, or F2.8, the resolving power will increase further while leaving the bokeh soft. I think that part of the appeal of this lens is that the images exhibit different resonances as you change the F value. I hope people will try it in a variety of situations and enjoy the level of expressive power that sets the SEL85F14GM apart from previous lenses.

SEL85F14GM

FE 85 mm F1.4 GM

SEL85F14GM