As well as being used in interchangeable lenses for Nikon SLR cameras, ED (extra-low dispersion) glass is also used in a range of other Nikon products and is a significant factor in their high levels of performance. This article describes ED glass and is interspersed with comments by Nikon corporate adviser Michio Kariya*, who was in charge of its development.
- *Michio Kariya:
Joined Nikon (formerly Nippon Kogaku K.K.) in 1967. Engaged in the development of optical glass materials and was also responsible for the development of related manufacturing technology and equipment. Appointed president of Nikon Imaging Company in 2001, president of Nikon Precision Equipment Company in 2002, president, CEO and COO of Nikon Corporation in 2005, and chairman of Nikon Corporation in 2010. Since 2012, he has served as corporate adviser to Nikon Corporation.
Chromatic aberration and optical glass
When white light passes into a triangular prism, a rainbow, as shown on the right, is produced. This illustrates how the refractive index of light varies according to its wavelength and is an instance of chromatic dispersion (hereafter, "dispersion"). Dispersion is also inevitable in interchangeable lenses, which use refraction both to concentrate and diffuse light. The phenomenon whereby color fringing appears on an image is known as "chromatic aberration."
Nikon currently uses at least 100 types of optical glass (hereafter, "glass"). These are superior to the glass used in windows in terms of homogeneity and transparency. Each type is imbued with its own particular optical characteristics, such as its dispersivity and its refractive index. They also all share a common characteristic in that if their refractive index is high, the dispersion is large, while if their refractive index is low, the dispersion is small. This characteristic is illustrated by the downward-sloping curve in the graph below.
If the refractive index of light were fixed so that it did not vary with wavelength, chromatic aberration would not occur. If this were represented on the graph, it would appear as a straight line with no slope. In other words, correcting for chromatic aberration may be restated as trying to construct an optical system that produces a line on the graph that has as shallow a slope as possible.
Although the dispersion in an interchangeable lens can be reduced by constructing it solely out of glass of low refractive index, this would require a composite lens with more elements than would be feasible. Conversely, constructing an interchangeable lens solely out of glass of high refractive index would only serve to increase dispersion. Hence, neither of these approaches is practical.
Accordingly, the method that is adopted for correcting chromatic aberration is to reduce the aberration by combining high-refractive-index, high-dispersion glass and low-refractive-index, low-dispersion glass. In other words, this method combines the steeply curve on the graph with the gradual curve to yield a moderately sloping curve.
In practice, however, it is not possible to correct for chromatic aberration across the entire visual spectrum solely by unifying these two curves, and chromatic aberration will persist in certain specific wavelength ranges. In order to correct this residual (secondary-spectrum) aberration, it is necessary to use glass with a dispersion that differs from that of ordinary glass in these specific wavelength ranges. This characteristic is known either as "extraordinary partial dispersion" or "anomalous dispersion" (in this article, the term "anomalous dispersion" will be used). Nikon has dubbed glass that possesses this characteristic "ED glass."
The development of a higher-performance telephoto lens
Chromatic aberration is greater at longer focal lengths. Hence, correcting for chromatic aberration is essential to improving the performance of a telephoto lens.
Nikon (formerly Nippon Kogaku K.K.) worked to develop a large maximum-aperture telephoto lens with a focal length of 300 mm and a maximum aperture of f/2.8 for the Winter Olympics held in Sapporo in 1972. This lens was intended to meet the demands of press photographers, who wished to take sharp pictures of indoor events at as high a shutter speed as possible. Nikon decided to build this telephoto lens using anomalous dispersion glass.
In January 1972, Nikon released the NIKKOR-H 300mm f/2.8, which was intended for use by media organizations. It was used to photograph a variety of different events at the Sapporo Olympics. The first such lenses that Nikon produced, however, were made using FK50 anomalous dispersion glass manufactured by the German company SCHOTT AG.
The young astronomer who could not tolerate color shift while viewing the night sky
When Michio Kariya was at high school, he observed the night skies using an astronomical telescope made by Nikon (formerly Nippon Kogaku K.K.). It bothered him that white stars that were slightly out of focus would appear yellow or green. While at university, he obtained a Nikon F single-lens reflex camera and noticed that the pictures that he took using a telephoto lens displayed slight chromatic aberration. This inspired in him a desire to make superior lenses.
Mr. Kariya joined Nikon in 1967 and was assigned to the Research Division of the Glass Production Department at the Oi Plant. His first job involved the development of infrared-transmissive glass. As this glass was non-oxide glass, a material on which Mr. Kariya had conducted research during his student days, the company made use of his knowledge and experience in this connection and put him in charge of the development of the composition of glass and the development of the required manufacturing equipment. The infrared-transmissive glass was delivered on schedule the following year (1968).
After the development of the manufacturing equipment, Mr. Kariya studied research reports that he found at the plant. Aware that the development of fluoride glass had ground to a halt, he began to work on it of his own accord. He read all the relevant literature that he could obtain—including material from abroad. His expectation was that the development of fluoride glass would be easy compared to the development of non-oxide glass (on which he had conducted research as a student) and he threw himself into a daily round of experiments.
What is fluoride glass?
The main constituent in glass is normally either silicon dioxide (SiO2) or boron trioxide (B2O3), to which various other substances are added so as to achieve the required optical performance. It was known at the time that fluoride glass—in which beryllium fluoride (BeF2) is used as the main constituent instead of silicon dioxide (SiO2)—possesses anomalous dispersion properties.
Although beryllium fluoride provides outstanding optical characteristics, its high toxicity makes it difficult to use in practice. Mr. Kariya tried many different compositions in an attempt to create outstanding optical characteristics without the use of beryllium fluoride. Time after time in the laboratory he would melt candidate substances in a small pot and then analyze the glass that was produced. On the ninety-ninth such attempt, he succeeded in synthesizing fluoride phosphate glass.
This is described as "December 1971: PC102—At Last, the Development of a New Type of Optical Glass" in the book "75-Year History of Nikon."
Repeated experimental melting
After completing the melting process in the laboratory, Mr. Kariya began conducting experimental melting using the furnaces at the Oi Plant. As these furnaces use highly valuable platinum, his boss warned him: "If you ruin that platinum, you'll have to pay for it out of your severance pay!"
Experimental melting was continued at the Sagamihara Plant, which was established in 1971. The very first attempt at melting at the Sagamihara Plant produced glass that was highly homogenous and was also of sufficient size.
Recalling that occasion, Mr. Kariya says: "Aside from the expertise that I had built up in the course of all my experiments, the fact that the conditions that day (such as the ambient temperature and humidity) were all favorable led to a successful outcome. It is no exaggeration to say that it was only by chance that we were able to produce glass of a reasonable size without any striae* in it."
In his second and subsequent attempts at melting at the Sagamihara Plant, noticeable striae appeared in the glass, and for over a year Mr. Kariya was unable to produce any decent glass. He did not give up, however. Although the reasons why the striae appeared in the glass were complex and extremely hard to fathom, Mr. Kariya remained confident. "Since we had succeeded once, there was no doubt that we would be able to replicate our achievement," he says. He investigated the problem both scientifically and theoretically, using an electron microscope to look at the sources of turbidity, which is related to the formation of striae. Over and over again he went through a process of analyzing an unsuccessful attempt at production, formulating a hypothesis, and then testing it.
Sections within the glass that have a thread-like or layer-like appearance. The refractive index of the glass fluctuates in these sections.
Setting up mass production of PC102
Mass production of PC102 did not begin soon enough for it to be used on the first NIKKOR-H 300mm f/2.8 lenses that were manufactured, and FK50 glass had to be used instead. Once mass production of PC102 had commenced, it was then used in the manufacture of NIKKOR-H 300mm f/2.8 lenses.
"We replicated the composition of FK50 and compared it with PC102. We also compared PC102 with fluorite. This enabled us to verify that PC102 was superior in many respects, such as stability, cost and ease of mass production," relates Mr. Kariya.
Other companies have announced products with compositions similar to that of PC102. None of them, however, have been able to rival PC102 in terms of stable mass production on such a large scale. This illustrates the fact that, to this day, no one else has yet been tenacious enough to devise the production methods or manufacturing equipment that Mr. Kariya developed.
PC102 was later dubbed "ED glass." Mass production now enables Nikon to use ED glass in binoculars and fieldscopes, as well as in interchangeable lenses. Glass derived from ED glass is also used in the projection lenses in IC steppers and scanners. ED glass has thus become an indispensable element in Nikon products.
A young engineer who aimed for the top
Mr. Kariya was 29 years old and in his fifth year at Nikon when he developed ED glass in 1971. He battled his way to new heights—overcoming difficulties that had proved too much for more senior engineers.
Looking back on that time, Mr. Kariya recalls that "as a development engineer who had only recently joined the company, I had yet to acquire sufficient experience and technique. Instead, I made use of theory and scientific analysis to overcome the difficulties that I encountered."
ED glass is used in approximately 40 of the 80 different models of interchangeable lens currently manufactured by Nikon. Through these products the vision of the young astronomer who wanted to make superior lenses has been passed to people all over the world.