What is a G Master? When you try this lens, you'll see the difference.
Osoniwa: The 24-70mm standard zoom is the most versatile lens format, and often the one that customers try first, so it forms their judgment of the good and bad points of a manufacturer's lens system. In that sense, you could argue that it serves as a billboard for the manufacturer, which is why our passion for the development of the SEL2470GM was extraordinary. In particular, since the G Master is based on the concept of bringing together high resolution and bokeh, we had to create a lens that lives up to this goal at any zoom range. With respect to the MTF (Modulation Transfer Function) as well, our goal from the design stage was to ratchet the performance up a notch to surpass all previous standard zoom lenses.
Bringing out resolution performance on par with a prime lens at any zoom position
Kuroda: In general, zoom lenses have difficulty achieving balance between corner resolution at wide-angle and maximum-aperture resolution at telephoto, but for the SEL2470GM, our goal was that there be no compromises at any zoom position or aperture setting. Our aim was to bring out resolution performance that is on a par with a prime lens in any position without compromising either the maximum-aperture resolution at telephoto or the resolution from middle to periphery at wide-angle. For example, you can get crisp resolution at the outermost edges in wide-open landscape photography, yet also get the resolution performance of a prime lens in 70mm portrait photography. The SEL2470GM is a lens that takes the all-around capabilities required for a standard zoom lens and really pushes them to an extremely high level.
Lens construction that takes full advantage of Sony's cutting-edge technology
Kuroda: The main point about the optical design of the SEL2470GM is that we managed to bring together overwhelming resolution with natural bokeh by taking full advantage of the superiority of the short flange back on the E-mount body and the newly-developed XA (extreme aspherical) lens element. In particular, because we were able to achieve extremely high surface accuracy at the 0.01-micron (micrometer) level in the XA lens element, it became possible to adopt it in the second lens group to create the highest magnifying power, and as a result, we were able to achieve unprecedented resolution performance and bokeh. Furthermore, adopting a direct drive SSM (Super Sonicwave Motor) with high stop position accuracy enabled us to use an internal focusing system and deliver a zoom lens with high-precision, high-speed autofocus as well as minimal zoom image shift and focus breathing when shooting movies.
Osoniwa: Also, in order to increase resolution across the entire zoom range, it's important to eliminate backlash from the mechanical parts. In the case of the SEL2470GM, there are multiple lens groups that move when you zoom the lens. How can you get the lens to move straight when zooming, and how can you get it to return back to its exact original position? Since the accuracy of the lens drive has a huge impact on image quality, we took the approach of experimenting with combinations of components one-by-one while creating the lens. It's also necessary to align the center of each lens, and performing the alignment tasks at each manufacturing step is very precise and careful work.
Best MTF for a zoom lens in the history of the α series
Kuroda: As you can see by looking at the MTF curve, the MTF stays very high even at maximum aperture, and maintains a solid performance all the way to the edges of the image compared to previous α lenses. I think we were able to achieve the best MTF for a zoom lens in the history of the α series. The 24-70mm lens is seen as the marquee of the brand, which is why I'm proud that we were able to create a good lens based on a design concept that allowed for absolutely no performance compromises.
G Master required the development of new alignment equipment
Kuroda: Relying only on conventional technology, it would have been difficult to ensure performance on a par with a prime lens across the entire zoom range. Therefore, for the SEL2470GM, we developed new equipment to allow us to drive the optical performance to a level of accuracy about five times greater than ever before. Using the new equipment to carefully align each lens individually, we were able to fully exploit the high potential of the design to bring together stunning resolution performance and bokeh, despite their inherent contradictions, and raise them to a higher level. You could say that the SEL2470GM has undergone not just a design evolution, but also a manufacturing evolution, which has positioned it one notch above the standard of previous zoom lenses.
New 9-bladed aperture for beautiful bokeh
Osoniwa: With respect to the aperture mounted on the G Master F2.8 zoom, on this occasion we redesigned the shape of the blades used in the 9-blade aperture unit in order to contribute to the goal of bringing bokeh and resolution together into their ultimate form. It's very difficult to come up with a design that ensures a clean circular shape that is well-balanced from the maximum to the minimum aperture opening, but by concentrating our attention thoroughly on just the parts that directly affect the shape of the bokeh, we were able to design an aperture unit worthy of the G Master label.
A direct drive SSM that excels in accuracy, size, and quietness
Osoniwa: To let our customers fully exploit the available resolution, which has been increased to the limit across the entire zoom range, we adopted a direct drive SSM with high stopping accuracy. There are also other types of actuators, but direct drive SSM, which has excellent speed and stopping accuracy, is the one that's most compatible with the highly optically sensitive design of this 24-70mm F2.8, and it enabled us to bring out the best focus performance. And in terms of size, we were able to come up with a design that exploits the space efficiency of the large-aperture lens frame by taking advantage of the very small size and high power characteristics of the direct drive SSM. We also thought its superior quietness would make people want to use it actively when shooting movies.
In fact, our requirements for installing the direct drive SSM specified that it must stop at the desired position, and must always exhibit the same performance. Moreover, the center of the lens must stay in alignment even when it is in motion. We configured the mechanical structure of the SEL2470GM to reduce backlash as much as possible to enhance stopping accuracy and smooth operation and minimize the tilt of the lens when stopped. Furthermore, in order to support the high-precision drive of the direct drive SSM, we successfully deployed highly rigid metal parts throughout the lens barrel, thereby achieving a very reliable barrel structure.
A lens design that's strongly video-focused
Osoniwa: Recently, the boundary between movies and still images has been disappearing, which means that the G Master can't compromise on movie shooting performance either. For that reason, in addition to the outstanding quietness of the direct drive SSM, an internal focusing system was adopted, reflecting a design that pays attention to movie shooting. We completely separated the position of the focus lens group from the position of the variator lens group, which is used to change the focal length, in order to smooth out wobbles during movie shooting and suppress fluctuations in the angle of view. I hope that customers will shoot some 4K videos on this lens so that they can more vividly experience the high quality of the lens.