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Precision Glass Mold (PGM) Aspherical Lenses

The aspherical lens has a curved surface that is not spherical and has the ability to correct aberrations. One type of aspherical lens that is particularly superior from the aspect of practical use and performance is the molded-glass aspherical lens, or the Precision Glass Mold (PGM) aspherical lens, as it is known at Nikon. In this Behind the Scenes story we talk with three engineers from the viewpoints of design, development, and manufacturing about the Nikon technologies that have made it possible to produce high-precision wide-aperture aspherical lenses.

Shigeru Sakuma
Manager, PGM Lens Sec., Aspherical Lens Dept., Tochigi Nikon Corporation
PROFILE:
After joining Tochigi Nikon in 1986, Sakuma spent 17 years developing short wavelength crystalline materials used in stepper optics. Since 2003, he has been working on the development and manufacture of PGM aspherical lenses at the Aspherical Lens Dept. Sakuma has been interested in photography since joining Nikon and currently enjoys using manual cameras and short-focus lenses. He is a fan of the photographer Shimpei Asai and the late physicist R. P. Feynman.
Yoshinobu Ogata
PGM Lens Sec., Aspherical Lens Dept., Tochigi Nikon Corporation
PROFILE:
Ogata has been at the Aspherical Lens Dept. since 2002 working on the development and manufacture of aspherical lenses with Sakuma. He has recently taken charge of the AF-S NIKKOR 14-24mm f/2.8G ED, AF-S NIKKOR 24-70mm f/2.8G ED, and other PGM aspherical lenses. On his days off, Ogata works up a sweat playing golf.
Takayuki Sensui
Manager, Design 2, 2nd Designing Department, Development Headquarters, Imaging Company
PROFILE:
Sensui joined Nikon's Imaging Company in November 2001 and has mainly been in charge of the optical design of interchangeable lenses. Each day he strives for greater precision in Nikon's products. Sensui has been involved in the development of the AF-S DX Zoom-Nikkor 17-55mm f/2.8G IF-ED , AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED , PC-E NIKKOR 24mm f/3.5D ED, and other lenses. His hobby is playing the drums in a band.

Molded glass aspherical lenses, vastly superior in cost and productivity

First of all, please tell us what an aspherical lens is.

Sakuma explains the advantages of molded-glass aspherical lenses over other methods in terms of manufacturing.

Sakuma:

As the name suggests, the surface of the lens is not spherical. "Aspherical" is the term generally used to refer to any lens that is not spherical. With spherical lenses, which have been around for years, the same curvature can be produced using a grindstone, but aspherical lenses cannot be made this way; the manufacturing process for aspherical lenses is extremely complex. PGM aspherical lenses, at the core of Nikon technology, are formed by heating optical glass until it becomes malleable and then pressing it into an aspherical die made of a special heat-resistant material.

Ogata:

There are basically three methods of manufacturing aspherical lenses. Besides the glass molding method, there is the "precision ground" method whereby the glass surface is directly ground with a grinding device, and the "hybrid" method where special plastic is molded onto optical glass.

What are the advantages of the PGM aspherical lens compared with the precision-ground aspherical lens and the hybrid lens?

Sakuma:

Since plastic is used in the hybrid lens, it is subject to environmental changes such as temperature and humidity fluctuations. However, such changes have a negligible affect on molded glass lenses. As for a precision-ground lens, the process of directly grinding the glass takes time and is therefore not suited for mass production. Since a molded glass lens is made by pressing softened glass into a die, it is possible to make a lens in a short time.

Sensui:

Another advantage of the PGM aspherical lens is its high refractive index. Lenses with higher refractive indexes perform better. Since glass that refracts well is hard and expensive, if it is ground, the grinding elements wear out quickly, making the production of even one lens extremely expensive. With molded glass, however, costs can be kept down, which is highly advantageous in the area of optical design where several elements are used in a single lens. Moreover, grinding marks appear in the bokeh areas on precision-ground lenses. This does not happen at all with molded glass. Nikon no longer uses the precision-ground method for making aspherical lenses for photography.

Sakuma:

The glass molding technique used to be more expensive than the hybrid method, but that difference has recently narrowed with small-diameter lenses. There is no apparent cost advantage yet with molded glass, but it may happen in the near future.

Ultra-high-precision Nikon PGM, achieved thanks to the hard work

When did aspherical lenses first come into use?

Ogata describes how the Nikon team took on the challenge of developing molded-glass aspherical lenses.

Sakuma:

In 1968, Nikon became the first manufacturer in Japan to bring aspherical lenses into practical use. It wasn't until the 1980s, though, that they became increasingly widespread. Precision-ground was the main method used in those days. . The glass molding method was first used for the AF Nikkor 18mm f/2.8D single focal length lens released in 1993. The glass molding method was developed and first used for the AF Nikkor 18mm f/2.8D single focal length lens in 1993. Wide-aperture molded glass was first used with the AF-S Zoom-Nikkor 28-70mm f/2.8D IF-ED lens released in 1998. I've heard that staff at Nikon's Sagamihara Plant at the time worked all night on this lens.

Sensui:

By the way, Nikon uses the term PGM to refer to molded glass. PGM is an abbreviation of Precision Glass Mold. The reason why the term "precision" is used is that we, the design and manufacturing team, work at micron precision levels. One micron is 1/1000 of a millimeter, and we also talk in units of 0.1 microns, which is even smaller by one decimal point. These aspherical lenses are made in extremely small units of measurement. The term PGM was used from the outset, because it's not just molded glass that we talk about but molded glass of ultra-high precision.

That's interesting. It must have been extremely difficult at the time to develop molded glass with such ultra-high precision.

Sensui describes his fascination with precision engineering while explaining why Nikon named this lens the Precision Glass Mold (PGM) asperical lens.

Sakuma:

The AF 18mm lens, in which we first used such glass, was small , so there wasn't a problem. The AF 28-70mm that followed was the first wide-aperture lens to use it. I heard that a lot of trouble went into mass-producing this lens. It was extremely difficult to achieve the precision demanded by the design division, and the pass rate for quality was low. After that, development work started on dual-surface aspherical lenses, and apparently these went through considerable trial and error at the time. But thanks to the tireless development efforts of our predecessors, we are able to work much better today. We are grateful to them.

Ogata:

There are differences in quality between lenses even when they are produced with the same die. Wide-aperture lenses in particular have greater differences than the smaller apertures. From the initial stage of development, Nikon undertook repeated challenges to meet the target of closing the gap with wide-aperture lenses. Furthermore, numerous processes are involved in the production of molded glass lenses, and I believe that major factors leading to the advent of high-precision wide-aperture PGM were the core technologies that Nikon had at related divisions such as glass materials, grinding and coating.