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.
 

   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.
 

   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.
 

   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.
 

   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.
 

   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.