3. HD PENTAX-DA16-50mmF2.8ED PLM AW
  4. Development Story

A large-aperture standard zoom lens assuring high-contrast, high-resolution imaging, even at image edges, and with minimal aberrations


A new-generation Star-series lens, perfected
in pursuit of the ultimate in image quality

A message from the development team

Developed as the latest model of the new-generation Star (★) series, the HD PENTAX-DA16-50mmF2.8ED PLM AW is a large-aperture standard zoom lens that has made tremendous progress by incorporating the best of PENTAX’s lens production technologies.

We brought together personnel from various development sections to talk about their enthusiasm for and devotion to the lens’s development and asked them to talk about its most notable features.

Product Planning
Tetsuya Iwasaki

Optical Design
Takahiro Nakayama

Mechanical Design
Makoto Niwa

Development background

(Iwasaki: Product Planning)

At the initial product planning stage of the HD PENTAX-DA16-50mmF2.8ED PLM AW, we set three goals that we intended to achieve:

We set these goals because we wanted to design a super-high-performance Star-series lens that not only optimizes the imaging performance of our latest PENTAX K-3 Mark III digital SLR camera, but also will be able to accommodate the more advanced APS-C-format digital SLR cameras of the future. We also wanted this new lens to be something to be cherished by users for many years to come.

Since it’s a large-aperture standard zoom lens — one of the lenses most frequently used by camera users — we had to pay meticulous attention to every detail. This meant that we had to anticipated all possibilities: the subjects and scenes to be captured; the way the user will handle and operate it; and the imaging performance at every focal length from close-up to infinity.

In the planning stage of this lens, we conducted research on how users felt about the smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM and what improvement they expect of a newer lens. We found the following negative results:

  • ・Low contrast and unstable resolving power at open aperture
  • ・Generation of a purple fringe under certain conditions
  • ・Slow, inaccurate autofocus operation

Our users pointed out a number of issues, including these. We felt a sense of crisis because this Star-series lens, which we expected to be the best of all interchangeable lenses, was starting to lose the confidence of its users. To regain their confidence in the Star series, we concluded that we had to completely redesign the optics and launch the next generation of Star-series lenses. We believed that it was possible to develop top-grade interchangeable lenses, which will please all users, by bringing together all the technologies PENTAX has accumulated and developed: optical design, design simulation, alignment, high-precision parts processing, actuator design, and mechatronics. For instance, we boosted our optical design, alignment and high-precision parts processing technologies using a high-precision simulation process for the development of four new-generation Star-series lenses (HD PENTAX-D FA70-200mmF2.8ED DC AW, HD PENTAX-D FA50mmF1.4 SDM AW, HD PENTAX-D FA85mmF1.4ED SDM AW, and HD PENTAX-DA11-18mmF2.8ED DC AW). We also designed a high-speed autofocus system and a high-precision focus control mechanism, using the latest mechatronics technology, for the development of the HD PENTAX-DA 55-300mmF4.5-6.3ED PLM WR RE. We focused on designing the best interchangeable lens — one that could optimize the imaging performance of the PENTAX K-3 Mark III — by making full use of PENTAX’s advanced lens production technologies. We intended to eventually create the finest APS-C-format digital SLR imaging system in PENTAX history.

Goal One: To develop a super-high-image-quality lens

As we began developing the HD PENTAX-DA16-50mmF2.8ED PLM AW as one of the next-generation Star-series lenses, we decided to completely review how we defined the PENTAX Star series. Our conclusion was that next-generation Star-series lenses should be:

  • ・Super-high-performance lenses assuring the best-possible image quality, with uniform sharpness and crisp image rendition from the middle of the image field to the edges, even at open aperture
  • ・Large-aperture lenses that extend the boundaries of photographic expression and creativity
  • ・Highly reliable, maneuverable lenses with high-grade barrel construction

Included in this definition are standards for image quality, which served an essential role in our pursuit of the ultimate image quality.

To meet these standards, we set very high development goals for all image-quality-related properties, including: high resolving power not only in the center, but also at edges; minimized chromatic aberration, distortion, flare and ghost images; and consistent imaging performance at all focal lengths. We then worked to achieve these goals by making best use of our high-precision simulation technology.

Goal Two: To provide high-speed autofocus operation

Looking at autofocus operation, we intended to design a much-faster autofocus system, because we thought that the autofocus speed of the smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM was seen as rather slow and unsatisfactory. We wanted the new lens to provide autofocus operation fast enough for the PENTAX K-3 Mark III’s high-speed continuous shooting. We worked not only to shorten the time required for focusing, but also to improve the precision and reliability (Goal Three) of autofocus operations. That’s why we decided to design a focus mechanism that incorporated a pulse motor linked directly to the lead screw — a rather difficult challenge to achieve with a large-aperture lens. We weren’t aiming for a small step forward, but instead wanted huge progress, so that users would clearly notice the difference. The result was an autofocus system which provided much-improved focus speed (approx. 2.2 times faster in the wide-angle range, or approx. 1.5 times faster in the telephoto range). Since the PLM (Pulse Motor) incorporated in this system is designed to be gear-free, we also succeeded in minimizing operation noise for stress-free autofocus shooting.

Goal Three: To design a highly reliable lens

For many years, this was a difficult goal to achieve, because we had to work on both mechanical and optical reliability. The pulse-motor mechanism incorporated in the new lens was highly reliable for durability, as proven with the HD PENTAX-DA 55-300mmF4.5-6.3ED PLM WR RE. Coupled with high-precision position control during autofocus operation, it was also highly reliable for autofocus precision.

Looking at optical reliability, the smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM suffered from minor inconsistencies in its optical performance. With the new lens, however, we achieved a better balance of imaging performance by minimizing the difference (or sensitivity variation) in captured images caused by the inaccurate positioning of optical elements. Coupled with our advanced alignment technology, we aimed at designing a highly reliable, dependable lens with minimal inconsistencies. We can make high-precision alignment by optimizing the position of optical elements on the production line, so each individual lens is consistent with all others, while still maintaining exceptional imaging performance and assuring excellent optical reliability.

As you can see, we made every effort to develop the most reliable lens in PENTAX history in every respect — from top-level imaging performance to flawless, high-speed autofocus operation. We believe that we had succeeded in developing a lens that assured great operational comfort and superb handling.

1. To develop a super-high-image-quality meeting the strict standards of the next-generation Star series

Never-ending pursuit of optimum image plane flatness

(Nakayama: Optical Design)

In optical design, we were most concerned about high-quality imaging over the entire zoom and aperture ranges. The first obstacle was to define the new-generation Star-series lenses. We then moved on to achieve high-quality imaging performance with uniform image plane flatness across the entire image field in the wide-angle range, where the spatial frequency of a subject is relatively high over a broad range. In the telephoto range, which often contains the main subject, we tried to work out a design which produced highest image quality as possible, particularly in the middle of the image field. As the result, the new lens produces finely detailed images across the image field in wide-angle settings, making it perfect for scene photography. In the telephoto range, it produces high-resolution images in the middle of the image field while creating a natural, true-to-life bokeh (defocus) effect in the surrounding areas, making it ideal for snapshots and portraits.

To optimize the precision of alignment and positioning

(Nakayama: Optical Design)

To improve optical performance at all focal lengths across the zoom range, we worked out a design which made it possible to adjust the image plane inclination and field curvature at several predetermined focal lengths. With previous zoom lenses, we had to balance optical performance at focal lengths other than the target length. However, the new HD PENTAX-DA16-50mmF2.8ED PLM AW doesn’t require such balancing act, making it possible to perform optimum adjustment over the entire zoom range. Also, we can now adjust optical balance with an automated regulator to perform consistent, high-precision adjustment operation and achieve a high level of optical performance.

To improve the stability of the optical axis

(Niwa: Mechanical Design)

For the mechanical components, we designed a totally new lens barrel. By adopting a structure that minimized the effects of bending caused by the extension of a zoom lens, we were able to constantly stabilize the optical axis. For instance, when you look down from a bridge at a stream flowing directly under you, or when you look straight up at the starry sky, the entire lens bends slightly due to its own weight as you shift your position. This sometimes results in the misplacement of the optical axis and a change in image plane flatness.

In the past, we employed a mechanism with a single roller used to roll over both the zooming cam groove and the linear movement groove. The new lens, however, features two separate rollers which fit the width of each groove (Diagram 1) to effectively minimize instability. To reduce unsteady movement along the optical axis, it also incorporates a structure which positions a roller and a ball bearing to apply sufficient tension (Diagram 1, 2). This prevents rotation torque of the cam ring from affecting zoom-ring operation, and assures effortless zooming without excessively large torque, while minimizing the generation of unwanted instability.

The new lens also features an optical element construction designed to effectively reduce the effect of changes in the lens’s inclination. It mechanically links two optical element groups positioned to the rear of the lens, while the third optical element group shifts between these two groups. This unique design stabilizes the optical axis regardless of the photographer’s shooting position.

To minimize aberrations for a bright, clear viewfinder image

(Nakayama: Optical Design)

We worked hard not only to reduce all types of aberration over the entire zoom range, but also to minimize the shift of chromatic aberration during focusing. To achieve this goal, we designed the lens for a perfect balance among all optical element groups, but most importantly with the focus element group. By incorporating one ED (Extra-low Dispersion) glass optical element, one anomalous dispersion glass optical element and two ED aspherical glass optical elements, chromatic aberration is kept to a minimum over the entire zoom range.

This minimized chromatic aberration is quite noticeable in the camera’s viewfinder, because of the reflex mechanism which lets the user view the optical image directly and intuitively. Just like chromatic aberration, the lens also minimizes distortion and optimizes color balance at the image-forming stage of an optical image, thus improving the quality of a viewfinder image.

To improve imaging performance in backlight — a PENTAX strong point

(Nakayama: Optical Design)

It’s a very difficult task to simultaneously improve both the color balance of an optical image and the imaging performance in backlight. We were able to accomplish this task, however, by applying a different type of lens coating to each optical element. The coatings specializing in reducing ghost images have been applied to the optical element surfaces prone to ghost images, while the coatings effective in improving color balance have been applied to the surfaces where ghosting is not an issue. The development of specialized lens coatings and the application of the most effective lens coating to each optical element are PENTAX strong points, which are clearly showcased in the HD PENTAX-DA16-50mmF2.8ED PLM AW.

2. To provide high-speed autofocus operation

Simultaneously achieving faster autofocus operation and higher image quality

(Nakayama: Optical Design)

To demonstrate a clear improvement in performance over the smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM, we set a primary goal of providing much faster autofocus operation in the HD PENTAX-DA16-50mmF2.8ED PLM AW. After reviewing a variety of lens types, we finally selected one lens type that would assure the high-speed autofocus operation and excellent imaging performance assured only by a Star-series lens. It would also make an excellent partner for the PENTAX K-3 Mark III, while maintaining a size small enough for current smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM users to handle without difficulty.

Compact, lightweight focus element group, made possible by a separation of optical element groups

(Nakayama: Optical Design)

For the smc PENTAX-DA16-50mmF2.8ED AL[IF] SDM, we chose a four-element-group zooming mechanism in order to downsize the lens size. In this mechanism, the second optical element group is used for focusing, because it provided the most powerful negative refraction. Since this type of lens tends to have relatively larger optical element groups, however, it’s not usually suited for high-speed autofocus operation. We also had to reduce various aberrations over the entire zoom range by shifting only a handful of the movable optical element groups.

However, we decided to incorporate a new type of zooming mechanism in the HD PENTAX-DA16-50mmF2.8ED PLM AW. By dividing the optical element group with the most powerful negative refraction in the previous zooming mechanism into two independent groups, we were able to reduce the size and weight of the focus element group. This achieved faster autofocus operation and reduced operation noise, while effectively compensating distortion and field curvature. By adding another movable optical element group, we could also reduce various aberrations more efficiently.

By placing aspherical optical elements in the front and back ends of the optical element group with the most powerful negative refraction, we minimized various aberrations over the entire zoom range. Also, by employing one ED (Extra-low Dispersion) glass optical element and one anomalous dispersion glass optical element and by adding two ED glass aspherical optical elements to the positive optical element group, we successfully minimized chromatic aberration over the entire zoom range.

Incorporation of a PLM(Pulse Motor)

(Niwa: Mechanical Design)

Thanks to the compact, lightweight lens design, we reduced the total bulk of the focus element group. This made it possible to incorporate a PLM (Pulse Motor) in the new lens.

The PLM is attached to the lens frame using a guide shaft — a structure that is prone to the effect of bending. We solved this problem by selecting a more rigid material for the lens frame. This means that the new lens can provide more stable optical performance, because it’s unaffected by the bending caused by its own weight.

The new lens also features a power-assisted focus mechanism, which detects the rotational angle of the focus ring and sends out electrical signals to drive the motor installed in the lens barrel. By pairing this mechanism with the slack-free PLM, we can assure high-precision manual-focus operation. Because this is a large-aperture standard zoom lens, intended for use with a variety of scenes and subjects, we paid the utmost attention to the accuracy of manual focus operations. Even the slightest rotation of the focus ring is detected by the mechanism for high-accuracy, error-free manual focusing.

3. To design a highly reliable lens

Simultaneously achieving a dustproof, weather-resistant body and exceptional operability

(Niwa: Mechanical Design)

Designed to be an AW (All Weather) lens with dustproof, weather-resistant construction, the new lens features nine sealing parts across the body to prevent the intrusion of water and dust into the lens interior. We carefully selected the sealing material and designed the sealing configuration to avoid the sealing parts from affecting the smooth operation of the zoom and focus rings.

SP(Super Protect)Coating

(Nakayama: Optical Design)

This PENTAX-exclusive lens coating is highly repellent to water and grease. Applied to the front surface of a lens, it not only prevents the adhesion of dust, water drops, oils and other substances, but it also makes it easy to wipe off any foreign substance that adheres to the lens. This special coating is also highly resistant to abrasion to keep the lens scratch-free.

Quality control

(Iwasaki: Product Planning)

All PENTAX interchangeable lenses are subjected to the most exacting quality-control inspections, not only in the design and development stages, but also during manufacturing. All manufactured lenses are carefully measured for various parameters, — resolving power, contrast, color tones and various aberrations — using the PENTAX-developed measuring instruments, so they are all given rigorous, objective numerical evaluations before shipment. Any lens which fails to meet any of the strictest in-house standards, by even the slightest of margin, is sent back to the line for retuning. This PENTAX-original quality-control system serves perfectly to maintain the exceptional quality of PENTAX lens products.

Final words

(Iwasaki: Product Planning)

We sincerely hope that, with this extraordinary new lens, PENTAX camera users will enjoy the experience of visualizing the final image based on what they see in the camera’s optical viewfinder, then releasing the shutter just at the right moment to preserve the scene or subject in a beautiful photograph. We also hope that this lens will remain your favorite for many years to come, and will enrich the photographic life of every photographer.

A large-aperture standard zoom lens assuring high-contrast, high-resolution imaging, even at image edges, and with minimal aberrations


A new-generation Star-series lens, perfected in pursuit of the ultimate in image quality