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Top > Tamron's 'Monozukuri' > An Inspiring Visit to a Plant Overseas > No.6 : Turning optical glass materials into a photographic lens

Kimio Tanaka

Kimio Tanaka graduated from the Photo Department of Tama Art School, Tama Art University and is a freelance photographer. He mainly specializes in taking photos of automobiles, but also covers a number of other genres, including portraits, landscapes and snap photos. His principal published works include Digital Ichigan Jotatsu Koza (Digital Single Lens Improvement Lecture), Digital Ichigan “ Kokanrenzu Nyumon (Digital Single Lens Interchangeable Lens Introductory Book) (both published by ASCII MEDIA WORKS), Digital Ichiganrefu - Shashin no Torikata (Digital Single Lens Reflex Camera Photo Taking Methods) (published by Gijutsu-Hyohron Co., Ltd.), Meisha Koyuroku (Acquaintance with Fine Cars) (published by Hara Shobo), and Meisha Tankyu (Searching for Fine Cars) (published by Rippu Shobo Publishing Co., Ltd.). He has held a number of photo exhibitions. At present, he is publishing information about cameras and lenses together with opinions about photos through his photo blog, “Photo of the Day,” and Twitter @thisistanaka.
The web page
http://www.thisistanaka.com/

“Turning optical glass materials into a photographic lens”

   The following four key processes comprise the work carried out until the optical glass lens elements are completed and delivered to the assembly process for interchangeable lenses.

   (1)Grinding and polishing of optical lens elements and manufacture of aspherical lens elements
   (2)Manufacture of lens components (Bonding, Partial assembly)
   (3)Final assembly (Fitting into the barrel, Sub-assembly)
   (4)Inspections and Adjustments, Cleaning

   Many optical glass lenses (glass materials) are combined to create one interchangeable lens. Various other parts are also necessary, but optical lens elements constitute by far the most important key parts of an interchangeable lens. The imaging performance and perceptive performance that I mentioned in blog No.2 are largely determined according to which optical lens elements are combined and how, and what the finish quality of these lens elements is.

   After the plan for a product is finalized, the personnel responsible for optical design commence the design work.
It is said that there are currently more than 200 types of optical glass lenses, and from among them, the designers select the optimum optical glass lenses for each product by carefully assessing the wide-ranging optical properties, such as the refractive index, dispersion characteristics and transmittance for light, thickness and size, etc., going on to combine these selected optical glass lenses.
   More specifically, the designers think about the aberration correction, resolving power, contrast, background blur effects, flare and ghosting, lens size and weight, cost, and manufacturability, etc. when making decisions regarding lens design.

   The design drawings are then completed, and according to their instructions, the plant workers grind and polish the optical glass elements and manufacture the aspherical lens elements to create one highly precise optical lens. This work is referred to as lens production.
   Please first take a look at the flowchart below.
   It shows the processes for creating a complete lens by processing the optical glass lens elements.
 

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   There are roughly two different routes taken for lens production. One route is to grind the optical glass material into a semicircular shape to produce a spherical lens element, and the other is to press-form the optical glass material at high temperature and pressure to create an aspherical lens element.

 

   Aspherical lens elements are categorized into two types: those with glass lenses as the materials (glass-molded aspherical lens elements), and those using plastics and glass lenses as the materials (hybrid aspherical lens elements). For the hybrid aspherical lens elements used for interchangeable lenses, a common production method is to bond a plastic aspherical lens element to a spherical lens element.
   Through the coarse grinding, fine grinding and polishing processes, certain shapes and geometries are formed to a certain extent for glass materials. Then, using a precision mold created in an aspherical shape, the glass materials are press-formed at high temperature and high pressure to produce glass-molded aspherical lens elements.

   The optical glass materials used as the materials for creating spherical and aspherical lens elements are not produced by Tamron. Just like many other camera and lens manufacturers, Tamron purchases them from manufacturers specializing in the production of optical glass lenses, as parts cut roughly into lens shapes.

   Tamron undertakes lens production mainly at its Namioka plant in Aomori, Japan and the Tamron plant in Foshan, China. The Namioka plant in Aomori is currently the only place manufacturing the aspherical lens elements (glass molded = GM) from glass materials, while the Tamron plant in Foshan produces hybrid aspherical lens elements, as does the Namioka plant.
 

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   This shows part of the manufacturing process for hybrid aspherical elements. After a resin material (plastic material) is formed into an aspherical shape using this equipment, it is bonded to a glass lens to produce a hybrid aspherical lens element. This work area is in a clean room environment to prevent the intrusion of dirt or dust onto the bonding surface. Anyone entering the clean room is required to put on dustproof workwear, including a dustproof hood hat and clean shoes. Naturally, I was wearing full dustproof wear myself when I entered the clean room to take a look. The room temperature is always maintained at a constant level. This is at Tamron’s Foshan plant.

 

   The manufacturing method for aspherical lens elements, especially for glass-molded aspherical lens elements, is top secret at every manufacturer, without exception. Special equipment is used, the level of technical difficulty is very high, and there is a lot of sophisticated knowhow involved. The extensive confidential knowhow could include how the ultra-precise mold is fabricated, how the temperatures and pressures are controlled at which levels in press forming, what the temperature should be upon mold release after pressing, and what the pressing time should be, among other aspects. Along with lens coating using a nano-structured layer, the eBAND coating process, and the fabrication method for precision molds, these constitute Tamron’s strictly confidential technologies and technical knowhow.

   There is no doubt that the further evolution and higher performance of aspherical lens elements will occur in the near future. They constitute a key component that will bring about rapid improvements in depictive performance, so with Tamron having possessed outstanding technological strengths in the creation of aspherical lens elements from the start, we can hopefully look forward to a bright future.

   For spherical lens elements, in contrast, orthodox methods and knowhow have been inherited from many years ago, with polishing and cleaning processes repeatedly performed to produce them. Lens grinding and polishing still require high-level techniques worthy of craftspeople.
   At the Namioka plant in Aomori, veteran lens artisans spend a great deal of time and effort to create each master test plate that becomes the standard for completing a polished lens element. The completed master test plates are sent to the Tamron plant in Foshan. Including the glass-molded aspherical elements and test plates, there are many things that only the craftspeople at the mother plants in Aomori are capable of creating at this point.

   Training is provided to the local staff at Tamron’s Foshan plant in China to foster their talent and improve their skills as craftspeople on a daily basis.
   At the same time, the automation of operations is also underway in areas where relying on artisan skills is not considered necessary. The main benefits of the automation of operations are ensuring stable quality and speeding up the processes. Regardless of the skill levels of individual workers, automated systems enable a large quantity of products with targeted quality levels to be manufactured in a short time. On the other hand, one shortcoming of automation is that, in view of the fabrication cost of automated equipment, it is not particularly suitable for small-quantity production.
   It seems that Tamron has a policy of valuing and maintaining artisan skills while also promoting automation where it is necessary.
 

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   After coarse or rough grinding, the next grinding process is fine grinding, which is carried out in a semi-automated way using dedicated fine-grinding equipment. Unlike the lens grinding and polishing processes in the past, there is no more scattering of abrasive compounds or the need for a worker to attend to all the procedures. Simply making adjustments for a number of grinding machines and setting the lens elements creates lens element surfaces with the specified accuracy. [At Tamron’s Foshan plant]

 

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   For the lens elements after fine grinding, transparent surfaces are created through this polishing process. For each of these polishing machines, as long as the adjustments are made in advance, it is only necessary to set the lens element materials, and they are automatically polished to the specified accuracy. The automation of the polishing process brings about more stable finish quality. After polishing is completed, the lens elements are quickly moved to the cleaning process. [At Tamron’s Foshan plant]

 

   At the same time, though, many processes requiring time-tested skills and knowhow comprising excellent craftsmanship still remain for lens polishing and production. Without very careful work with close attention to detail, it is simply not possible to create products with superb quality.
   For example, abrasive compounds are used for lens element polishing, and unless full cleaning is performed upon the completion of each polishing process before moving on to the next process, it is not possible to create a lens element surface with high accuracy. (Purified water is used as a cleaning liquid.) In addition, because a cleaned lens is prone to reacting with the oxygen in the air, possibly causing an unwanted change in its color (lens discoloration), each lens must be handled with the utmost care.

   As shown in the flowchart above, lens grinding and polishing is essentially about gradually smoothing the surface roughness, and primarily consists of the following three steps. The specified curvature is gradually achieved by undertaking these grinding and polishing steps.
   The first step is coarse grinding, after which the surface roughness is approximately 4-10 μm, and this is smoothed to around 0.2 μm after fine grinding. 1 μm equals 0.001 mm. Then, after the finishing process of polishing, the unevenness on the lens surface is down to about 0.002-0.015 μm, with a transparent photographic lens now emerging.

   The key tool for checking whether the curvature of the polished lens element matches the design value is the test plate lens. As already mentioned, each of the test plates is an original, handcrafted glass lens. Craftspeople with special skills and knowhow carefully create these test plates, and there are a number of such veteran artisans now working at the Namioka plant in Aomori.
   The quality of the test plate may sometimes affect the accuracy of mass-produced lenses. It is often the case that these artisans create test plates not only for their own company, but also in response to orders from other companies. A little-known fact is that Tamron is actually a well-established manufacturer, famous for its creation of original test plate lenses.
 

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   A large quantity of water and abrasive compounds is used in the polishing processes. This means that it is inevitable that stains and dirt begin to appear in the workplace. At the same time, eliminating dirt and dust while carrying out the polishing work itself is required as much as possible. During the polishing processes at Tamron’s Foshan plant, I saw a number of workers carefully mopping the floor to remove dirt and stains, as shown in this photo. These little acts of care and attention to detail are in fact directly linked to the levels of quality of the final products. [At Tamron’s Foshan plant]

 

   After the completion of the specified lens polishing and cleaning processes, a coating is applied to the lens element surface. The coating processes are performed by paying the utmost attention to excluding any dirt or dust.
   There is also a process of grinding the circumference (edges) of each lens element so that the lens’ optical axis is positioned precisely in the center of the lens diameter. This is called centering, and the lens circumference is accurately ground by judging where the sole optical axis should be located. Nowadays, this has been turned into an almost fully automated process, but it is still a very delicate and rigorous process, with even minute misalignments not allowed. (Sometimes this process is completed prior to coating.) As part of this centering process, the end face shapes are accurately created to ensure that the lens can be firmly anchored in the designated lens barrel.
   Centering is carried out for aspherical lens elements as well, but the tricky thing is that many optical axes exist for an aspherical lens, which requires the work to be performed with even more delicate care and attention to the smallest details.

   As necessary, the two lens elements are bonded together using a special adhesive. This process also requires delicate care to ensure that not only dirt or dust but also small air bubbles do not find their way onto the bonding surface.
   For each of the lens elements after coating and bonding, all the end faces on the circumference are coated in black using India ink. This process is called India ink coating or edge coating.
   This is done to prevent any incidental light from reflecting inside the lens. Depending on the lens, India ink coating is performed so as to very minutely go over or stick out a little from the edge. Appearing deceptively easy, it is actually a very difficult and demanding task.
 

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   When you visit Tamron’s company website, you will find a feature section titled “Factory Tour PRESENTED BY TAMRON : Steps in making lenses for DSLR & Mirrorless cameras,” which is a very well thought out visual introduction to the wide-ranging processes until an interchangeable lens is completed, using a lot of nice photographs. I would highly recommend that you take a look at this section for reference. (http://www.tamron.co.jp/monozukuri/tour/#/)

 

   In the flow of lens production work, inspections (performance checks) are conducted repeatedly before moving on to the next process. This is work that is very low key, with a lot of patience needed, but it is not possible to avoid these careful checks to ensure excellent quality.
   Visual inspection with the worker’s eyes is also conducted to undertake a rigorous inspection of the external appearance, with each lens element held up to bright light. Even a very minute blemish or scratch on the surface requires the element to be removed as defective.
With the lens elements finally completed in this way, they are transferred to the lens assembly processes.

 

 

   The story on lens production this time around was admittedly a rather long one.
   In the next article, I will provide a broad overview of Tamron’s Foshan plant and guide you through the plant. Please look forward to the presentation, which will show what kind of place Tamron’s plant in China actually is.
 

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   At a market in the city of Foshan, where the Tamron plant is located. The city adjacent to Foshan is Guangzhou City, which is famous as a top food city. I saw a great variety of foods at this market in Foshan, including fish, meat, vegetables and dry goods. (In fact, I even saw “foodstuffs” such as snakes and turtles that were alive!) This is a fish store. They may be a husband and wife or an older brother and a younger sister, but anyway, despite my asking them, “Excuse me, is it OK if I take a picture?” with gestures, they both seemed to feel shy and wouldn’t look toward me.
   SP 45mm F/1.8 Di VC USD (Model F013), Aperture-priority automatic exposure (F/3.5, 1/50 second), -0.3 EV exposure compensation, Auto ISO (ISO 1100)

 

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   The city of Foshan is a very safe place security-wise. Even while I was strolling by myself at night along back alleys that initially appeared a little dubious, I never really felt afraid, not even once. In bustling areas, you even see children like these playing, full of energy. When I called out, “Hey, I’m gonna take a picture of you, OK?” (in Japanese, of course), these Chinese kids flashed the peace sign (V sign) all at once, just like the kids in Japan. Oh well… Even with a high-power zoom lens, if it has a built-in vibration compensation (VC) mechanism, you can casually take snapshots in low light at night, although a sudden motion blur just cannot be concealed, even by the superb VC system.
   28-300mm F/3.5-6.3 Di VC PZD (Model A010), Aperture-priority automatic exposure (F/5.6, 1/25 second), -0.7EV exposure compensation, ISO 12800

Kimio Tanaka

Kimio Tanaka graduated from the Photo Department of Tama Art School, Tama Art University and is a freelance photographer. He mainly specializes in taking photos of automobiles, but also covers a number of other genres, including portraits, landscapes and snap photos. His principal published works include Digital Ichigan Jotatsu Koza (Digital Single Lens Improvement Lecture), Digital Ichigan “ Kokanrenzu Nyumon (Digital Single Lens Interchangeable Lens Introductory Book) (both published by ASCII MEDIA WORKS), Digital Ichiganrefu - Shashin no Torikata (Digital Single Lens Reflex Camera Photo Taking Methods) (published by Gijutsu-Hyohron Co., Ltd.), Meisha Koyuroku (Acquaintance with Fine Cars) (published by Hara Shobo), and Meisha Tankyu (Searching for Fine Cars) (published by Rippu Shobo Publishing Co., Ltd.). He has held a number of photo exhibitions. At present, he is publishing information about cameras and lenses together with opinions about photos through his photo blog, “Photo of the Day,” and Twitter @thisistanaka.
The web page
http://www.thisistanaka.com/

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