Realizing Cinelens Weighing Less than 1kg

FUJIFILM's FUJINON lens is best known as the leader in the broadcasting industry with its cutting edge technology, but the maker has been making quite a stir also in the cinema world with several FUJINON cine lenses in recent years.

In 2009, FUJIFILM announced the world's first 4K cine zoom lens.
In 2013, they announced the ZK Series, defying common sense with the idea of attaching a drive unit to a cine lens, so that the lens can also be used in TV production site.
And in 2017, they announced the MK Series, targeting cinematographers with limited budget, but do not want to compromise on the image quality. FUJINON cine lens has been known as compact and lightweight, but even then, the existing models weighed at least 2kg. Realizing a lens that weighs less than 1kg while maintaining the same optical performance must have been not easy.

We interviewed the Cine lens team at FUJIFILM about the development of MK18-55mm and MK50-135mm.

Noguchi (Product planner): The product planning team decided on the main spec of the lens when the development started. The goal was to realize a lens designed for Super 35mm / APS-C sensor, and it is ultra-compact and lightweight, weighing less than 1kg. In terms of compactness, we requested the designer for 85mm front lens diameter and 82mm filter size with 0.8M gear pitch.

We decided on the 82mm filter size as we wanted the filter size that is readily available in most retailers. You would have to go to specialty store for 95mm, but 82mm filter are readily available in most retailers.

Yonezawa (Optical designer): The demand from the product planning team was to keep the same optical performance as other FUJINON cine lenses, but it had to be less than 1kg. On top of that, they requested the operability to be same as higher end models. I thought the hurdle that we had to get over was really high. Even the XK lens weighs 2.4kg without the drive unit, and that was the most compact and lightweight lens in our lineup. I thought to myself a lens that weighs less than 1kg is impossible to realize.

Kawamura (Mechanical designer): Since the design phase, we focused on the goal that it would weigh less than 1kg. We set target values of each mechanical and optical parts and how each should weigh. With our first deisgn, we realized that it was not possible to reach the target value due to restrictions of optical design, but by considering various elements such as increasing the ratio of mechanical parts and lowering the ratio of optical parts, we were able to realize the final value of 980g.

Yonezawa: The front lens was originally 90mm in diameter, but we had to reduce the size to less than 60mm to have common filter size between the two MK lenses. In my past project, I was assigned to design an 8K lens and I faced that exact same problem back then. I applied the zoom construction of that 8K lens to the MK lenses and I was able to make the front element smaller.

In addition, we used lots of ED lenses (special optical lens) so chromatic aberration is suppressed meanwhile the size and weight reduction are realized. We used 4 ED lenses for MK50-135mm and 8 ED lenses for MK18-55mm. When the lens size is reduced, the aberration from each element become greater. We used greater number of ED lenses so the aberration can be suppressed as much as possible for each element, realizing the high optical performance in this size.

Kawamura (Mechanical designer): In terms of mechanical design, smaller lens meant smaller lens barrel. So the focus unit, zoom unit, and aperture unit also had to be smaller in size to fit in that limited space. Our challenge was to how we can put each unit while maintaining a good balance.

In addition, we used plastic for exterior and lens holding parts in order to cut the weight. I think we achieved the final product to have the similar appearance and texture of aluminium. We've had people comment that "it does not look plastic." We worked really hard to achieve the exterior. We examined the paint compatibility, and added glass fiber for durability. The texture of glass fiber vary depending on the length, so we experimented a lot to realize the final product.

Toyama (Optical designer): The use of plastic also meant a challenge in terms of optical design. Plastic allow you to cut weight, but they are vulnerable to temperature change. Our goal is to achieve the same high optical performance of FUJINON cine lens even though the lens barrel is made of plastic.

To overcome the problem, we analyzed the combination of lens shape and glass material and how the focal point change due to temperature can be controlled. We achieved the quality that is on par with that of the aluminum exterior.

Noguchi: We need to take great care of temperature change. The lens' focus performance must remain unchanged even if there is a temperature change. The lens is designed assuming that it will be used in harsh environment.

Sakai (Designer): As the flagship models, HK and ZK Series appealed the robustness of solid metal. The lens barrel is milled from aluminum. The black part is black anodized surface, and the green part is treated with green alumite.

The MK lens is an entry model. Plastic is used to achieve lightweight, but the design concept is the same as other FUJINON cine lenses. The appearance of black alumite and green alumite is also achieved for MK Series. The difficult part was to realize the alumite appearance on the plastic exterior. How can we achieve the bespoke black alumite shine? How can we achieve the highlight with paint? We repeatedly experimented the toning to achieve the final appearance.

We assumed that the lens is not only for rental, but also for purchase. So we wanted to deliver the joy of owning the lens with the look and feel. We try to make it as close as aluminum.

Kakinuma (Optical engineer): We adopted a mechanism that can easily achieve the quality standard during the adjustment phase. In order to improve resolution, conventional FUJINON cine lens has been assembled by craftsmen and carried out difficult adjustment. Introducing a revolutionary adjustment mechanism from the MK lens, it became possible to automatically determine the optical performance beforehand so that adjustment work for performance can be completed in a very short time.

For the production process of MK lenses, we wanted to introduce new adjustment mechanisms and new facilities. Everyone involved in the process, from optical designer, mechanical designer, to the staff at the production site shared their ideas.

Sasaki (Optical engineer): The precision of cine lenses parts are in micron scale. The demand for precision is especially high for the high resolution lenses such as 4K lenses. In order to construct a lens, 20 or more glass elements are needed. Each element surely needs to be precise and the final construction needs to have a good balance. Each lens is custom made essentially, and this was how we used to produce conventional cine lens.

MK lens has no compromise in terms of its performance, but we revised the production process by realizing shorter leadtime. We cannot tell you in detail, but we introduced a mechanism that can predict the final proformance at the early production stage, so the high quality lens can be realized in much shorter period of time.

Yonezawa: The focal shift is inevitable for still lenses, because a typical still lenses combine zoom and focus group into a single group to achieve the compact size. On the other hand, Cine lenses use separate zoom and focus group. It makes the lens larger in size, but the focus group's position remains unchanged while zooming. With this construction, focal shift can be minimized while zooming.

The lens construction of MK lens is same as the higher end models such as ZK and XK Series. The only difference is the lens shape, size and position. The focus group that remain unchanged in its position is found in front of the zoom group. In the rear of the focus group are the aperture and another group that remains still in the same posistion. With this construction, focal shift is minimized while zooming.

Yonezawa: Breathing also has to do with the lens construction. As mentioned, Still lenses use same focus and zoom group, so inevitably zoom changes when trying to change focus so the angle of view changes. Zoom and focus are independent for MK lens. Zoom group does not move when changing focus, so the breathing can be minimized.

Kakinuma: Errors of each part build up to optical axis shift. MK lens has adjustment mechanism that adjust the error of each lens and moving groups. Conventional cine lens and broacasting lens also use the same mechanism, so we put that into MK lens.

Tanaka (Optical designer): The colors can be quantified in numerical values with CCI (Colour Contribution Index). We adjusted the numbers to that of the higher end models. In order to adjust the color, light transmittance per wavelength need to be set to a certain value. The value depends on the lens material and coating, but we took all that into consideration to have the same value as the higher end models.

Kawamura: The torque is same as other FUJINON Cine lens. The focus ring can rotate fully up to 200 degrees, which is same as the XK Series. If the angle is narrower, a focus adjustment is hard to achieve.

Noguchi: HK Series can rotate up to 280 degrees. This is because, we assume the HK Series as two-men operation lens, so the angle is wider. We designed the ZK, XK, and MK Series assuming that it would be one-man operation. 280 degrees is too wide to rotate singlehandedly, so we interviewed cinematographers all around the globe and concluded that 200 degrees is the optimal angle.

Kawamura: In terms gear pitch, all of our lenses use 0.8M. Same mechanism is used for iris ring for seamless operation throughout the range.

Hayashi (Product planner): We spent much of our time discussing about the torque. Originally the focus ring toruqe was heavier, but we decided to make it lighter in the end, as it would be easier for one-man operation. Unlike the higher end models that used metal, plastic was used so variation in dimension was hard to overcome.

In regards to the feel of the ring, we used the 3D printer and made various patterns and checked each operability. We also asked cinematographers for advice and reflected the feedback on the design. I acutally went to the factory to check on the torque in the production.