After the launch of the PENTAX K-1, our long-expected 35mm full-frame digital SLR camera, the biggest issue we faced was the limited lens selection. We very much knew that we needed to expand our lens lineup, but we were uncertain about where to start to solve this imminent problem. We finally decided to begin product development assignment by prioritizing some lenses.

We wondered what users were expecting from the PENTAX K-1; we concluded that they demanded the high image quality made possible only by a 35mm full-frame image sensor — its exceptional resolving power, truthful expression of gradations and outstanding imaging performance in the high sensitivity range. Our survey of actual PENTAX K-1 users backed up this assumption.

Based on this finding, we began development of three large-aperture zoom lenses — ultra-wide angle, standard and telephoto models — to form the core of our high-resolution, full-size imaging system. Our next step was to consider the development of large-aperture single-focus lenses, which would provide even better image quality and imaging power. At first, we wanted to provide full-frame single-focus lenses with angles of view that were different from those provided by existing models, and even initiated a project in this direction. These included an 85mm F1.4 lens, which was listed on our lens roadmap. After conducting research on the types of lenses our users desired, however, the most sought-after model was a 50mm lens — a standard single-focus lens familiar to all users since the days of film photography. There were several other options, but since this was what users most wanted, there was no reason to ignore the demand. So priority was placed on the development of the HD PENTAX-D FA^★50mmF1.4 SDM AW.

PENTAX Star-series lenses are positioned as premier-quality lenses; they are roughly defined by their excellence in three categories: exceptional image quality, large aperture, and a lens barrel design with excellent strength and outdoor protection performance.

We decided to make a thorough review of basic specifications worthy of Star-series lenses in order to ensure that they would be comparable with future camera bodies, which will continue to evolve and provide better imaging performance and higher resolution. To be more precise, we fine-tuned overall lens performance, especially by drastically raising the resolving power standard, while simultaneously reviewing factors such as MTF (Modulation Transfer Function) characteristics, chromatic aberration, distortion, and a true-to-life bokeh (defocus) effect. Of course, we also set high standards for the large aperture and high-grade lens barrel with excellent strength and operability, based on the values today’s users assign to us.

While we demanded that existing Star-series lenses had the highest performance possible at the time of their launch, we believe that we can set the standards even higher for future Star-series models, assuring the quality that PENTAX’s top-of-the-line lens series deserves.

As the Product Planning team explained, we wanted to optimize the resolving power — particularly the resolution of high-frequency components — while thinking about the specifications of new-generation Star-series lenses. This will also enhance the imaging performance of low-frequency components, and improve the contrast of captured images, helping the lens to deliver a sharp, crisp image description.

For the lens construction, the modified Gauss-type optical design, commonly used for large-aperture single-focus lenses, somewhat affects the image description at and around open aperture — both numerically and in actual images — because of residual aberrations.

Of course, there are some lenses which take advantage of the image description typical of such lens construction. Because our goal was optical performance setting a new benchmark for Star-series lenses, however, we found that, if we used the conventional modified Gauss-type optical design alone, it would be difficult to correct the aberrations resulting in a linear bokeh effect and flare. By adding large-aperture correction lenses to the front optical element group, though, we were able to effectively compensate spherical aberration causing flare, Sagittal coma flare causing a linear bokeh effect, and field curvature. This meant we could assure extremely high imaging performance from the center of the image field to the edges.

Also, by using the rear optical element group (which had already reached a much higher level of precision) as the focus lens, we could assure solid focusing operation without affecting optical performance or the overall balance of aberration compensation. At the same time, this also effectively compensates for the field curvature often generated at a close range, greatly improving imaging performance in close-range photography. Three Abnormal Dispersion optical elements also helped reduce chromatic aberration, while dramatically improving the MTF characteristics of high-frequency components. Optical performance can be enhanced by increasing the number of optical elements, and the correction lenses incorporated in the front optical element group definitely served this purpose. On the other hand, interchangeable lenses have certain restrictions due to the mount size. This meant that there was a limit to the number of optical elements we could add to the rear optical element group. By adding an aspherical optical element to the rearmost lens, we achieved a high level of aberration compensation (a high level of imaging performance) without increasing the number of optical elements.

We also adopted several of the state-of-the-art coating technologies available to us: Aero Bright Coating II, which drastically reduces surface reflection of incoming diagonal light thanks to its outstanding low-reflectance properties made possible by nanotechnology; and HD Coating, which greatly reduces the average reflection rate over the entire visible ray range. Even in the demanding shooting conditions caused by complex lighting such as backlight, we effectively minimized flare and ghosting.

Lenses with a distinctive style can be interesting. This time, however, we were assigned the task of developing an exceptional standard lens. So we aimed at developing a superb lens which could perform brilliantly in all photographic applications.

This lens was developed to become the best autofocus single-focus standard lens, one that provides superb optical performance and image quality unparalleled in PENTAX history. When we actually placed the optics that resulted from the optical design into a jig, we were captivated by the exceptional descriptive power. Based on our experience, we were confident that we could move ahead into mass production without concern. But when we closely examined the structure inside the lens barrel and other mechanical elements, however, we encountered some simple but pretty serious problems.

As the Optical Design team explained earlier, the lens’s optical performance depended on a delicate balance. Since the correction lenses we needed to provide the expected descriptive power were fairly heavy, the lens construction shifted the center of balance toward the front end. This caused slight bending of the lens barrel, which misaligned the optical axis and spoiled its descriptive power.

To solve this problem, we developed a unique structure in which the support for the correction lenses was sandwiched between resin and metallic components. These two components, which acted like the beams and foundation in building construction, prevented bending. To reduce the added weight which this support structure created, we looked carefully at the material used for the resin component. After repeated simulations, we succeeded in reducing weight without sacrificing structural strength.

The focus mechanism also required some creativity. The focus lens group needs a bit of space between the individual optical elements, so that lens extension is smooth and flawless. But for this lens, these small spaces might affect optical performance. We installed a mechanism to apply a small degree of tension to the focus lens group drive unit, improving the positioning accuracy of all the optical elements in the limited spaces and preventing the misalignment of the optical axis. Applying too much tension would in turn demand greater torque to drive the focus mechanism, but we eventually found the optimum balance between the high-precision positioning of the optical elements, and the smooth extension of the focus lens group.

To see the high-precision construction, just look at the mount ring, which is fixed to the lens barrel with the highest number of screws ever used.

This all shows the improvements we needed to make even in insignificant places, revisions we wouldn’t need to make for conventional lenses. By taking on and resolving all these small issues one by one, we succeeded in developing a lens barrel that assures uniform imaging performance in all circumstances.

Some users might question the durability of such a high-precision optical structure. But they don’t need to worry at all. This is a PENTAX Star-series lens, one in which every single detail has received the utmost attention, not only in assembly and operational precision, but also to durability over many years of use.

After reviewing several options for the lens drive motor, we developed a new SDM (Supersonic Direct-drive Motor) mechanism. This incorporated a ring-type supersonic motor, which could efficiently respond to the weight of the focus lens group being shifted, and the positioning accuracy of optical elements for pinpoint focus. Installed in the lens barrel are a position sensor which detects the position of the focus lenses steplessly, rather than in steps, and a sensor which detects the SDM’s rotation speed. Obtaining position and speed data simultaneously optimizes the balance between autofocusing speed and focusing precision, despite the lens’s large open aperture of F1.4 and its very shallow depth of field.

The new SDM installed in this lens delivers torque 7.5 times greater than that of the more compact supersonic motor installed in the conventional SDM mechanism incorporated in other DA-series lenses.

Since there is a priority on torque, the focus ring may require a little extra effort to turn during manual focusing. But it assures very little backlash and provides comfortable, reliable focus operation.