People often wonder if Leica
makes their own glass for the manufacture of their lenses. They used to, as a matter of fact, they
operated their own glass research lab where they developed many of the exotic
types of glasses that are used in their lenses today. However, for a relatively small company, that
proved to be unsustainable, and they closed the lab as well as their glass
manufacturing operation.
The Leitz Glass Laboratory,
which operated until 1989, was the driving force of Dr. Gustav Weissenberg, of
the Mineralogical Institute of Marburg, Germany. The Institute grew crystals, and
Weissenberg’s idea was for it to create optical glasses as well as to meet the
demands by optical designers for high refraction glasses to correct
aberrations. He wanted to replace the
highly radioactive thorium oxide in existing glasses with an oxide free of
thorium. So, in 1949, the Leitz brothers
established a glass laboratory within their company to pursue Weissenberg’s
research.
After lengthy experimentation
the glass laboratory discovered that lanthanum oxide offered the best chances
for success. They found that glasses
could be created with a refractive index of n 1.7. However, such glasses could not be made in
large quantities because their strong propensity for crystallization made it
difficult to prevent crystal grooves, or striae. Other substances, such as zirconium oxide
yttrium oxide, and tantalum oxide, had to be incorporated to produce more
stable high-refraction glasses in large numbers. Leitz then granted the German company Schott
a license to produce the new LaK 9 glass exclusively for Leitz.
Meanwhile, the Leitz optical
designers wanted to design lenses that were even faster than f/2, and the
dreamed of doubling the speed to f/1.4.
This required glasses with refractive indices close to n 1.8, and for
this they had to find the right technology.
Evaporation at the surface of the molten glass while the high
temperature in the crucible is being lowered can easily cause striae, which can
only be prevented by stirring to produce constant blending of the molten
mass. In order to accomplish this,
technicians had to incorporate interference factors, so that one oxide would
prevent another from crystallizing.
Laborious experiments led to three new types of Leitz optical glass,
with refractive indices of ne 1.80 and ne 1.82 and a dispersion value of ny
45. These new types of glass made the
design of the 50mm f/1.4 Summilux lens possible
Platinum crucible
Stirring rod
Pouring of molten glass
But for glass types with
indices of refraction higher than ne 1.8, it was difficult to obtain enough
chemicals with sufficient purity at a reasonable cost. At the time, one kilogram (2.2 lbs.) of 99.99
percent tantalum oxide cost the equivalent of nearly $200.00.
In 1966, Leitz introduced the
50mm f/1.2 Noctilux, which incorporated aspherical surfaces. Then years later, it was producing the 50mm
f/1.0 Noctilux , which did not involve aspherical surfaces. This was made possible by still another new
type of optical glass: one with a higher zirconium oxide content.
The company was seeking a
lens with a maximum aperture of f/1.2, for which optical glass with a
particularly high index of refraction was necessary. Ultimately, it wanted to double the maximum aperture
of the 50mm f/1.4 Summilux lens to f/1.0.
The higher zirconium oxide content permitted production of the 900403 glass with a refractive index of ne 1.9005 and a dispersion value of ny
40.
Comparison of optical glasses
The large circled dots indicate glasses used in Leica lenses, the small circled dots indicate Leica glasses not currently in use. The plain dots indicate glasses from other manufacturers
These developments were not
without difficulties, however. The new
lenses utilized glass with a very high melting point of nearly 1600 C (2912
F). That came close to the melting point
of the platinum crucibles. Also, the
shape of the stirring tools in relation to the viscosity of the molten glass
was important for the homogenization of the glass. So, the temperature of the melt in the
crucible had to be lowered considerably before it was poured out. Workers then had to cool the slabs at a
carefully controlled rate over 10 to 12 days and nights in order to prevent
molecular tension. Furthermore, the
glass was still quite susceptible to crystallization.
Another area of investigation
at Leitz was correction of chromatic aberrations in apochromatic lenses with
long focal lengths. Fluorite, grown in
single crystals, possesses such anomalous partial dispersion for correction of
the fourth color in optical glass. Leitz
competitors liked to use these calcium fluorite crystals for lens elements in
telephoto lenses, but Leitz did not regard them as suitable for photographic
lenses because they have very poor shape retention. Also, because of their low index of
refraction of only ne 1.43, the fluorite crystals require strong curvatures,
which are disadvantageous. Leitz
preferred to search for a true glass with an amorphous structure.
The company discovered that
the fluorites remained stable in metaphosphate suspensions. By incorporating several fluorites, it was
able to optimize the proportions so as to avoid creation of striae during the
cooling process. Eventually it succeeded
in developing a true glass with anomalous partial dispersion and a refraction
index greater than ne 1.544. With that,
Leitz became the first lens maker to manufacture such lenses. The 800mm f/6.3 Telyt-S was such a lens [see
blog article “Dancing Bear and his Magic Lens”]. It attracted a great deal of attention at the
1972 Olympic Games in Munich.
A few years later, with the
addition of titanium oxide, Leitz was able to produce glass with an anomalous
partial dispersion value of ny 66.6 and a refraction index of 1.544. This glass was used in the 180mm f/3.4
APO-Telyt, which ranks to this day as one of the best lenses ever made. Since then, the company has made a number of
lenses with apochromatic correction, and it is creating new ones regularly.
Overall, the Leitz Glass
Laboratory developed 35 new glasses from 50,000 experimental melts. For a time, these glasses were used
exclusively in Leica lenses. Without the
laboratory, in fact, most of the modern lenses of the period 1949 to 1989 would
not have been possible. Because of its
work, the company produced up to 10 metric tons of glass a year, partially
because of close communication between glass researchers and lens designers. Other large glass manufacturers were often
behind Leitz/Leica because optical glass is of only minor importance to
them. Phototropic glasses, construction
glasses, television tubes, baby bottles and the like are their real
moneymakers. When these companies do
undertake to develop optical glasses, it is only for those for which they
anticipate a large demand. It was the
exotic wishes of the Leitz Company that permitted it to develop the glasses it
did during the 40 years it held sway in this area.
Once it was possible with new
technology to derive maximum performance from optical glasses, however, Leica
closed the Leitz Glass Laboratory. The
growing number of glass manufacturers and new methods of processing glass and
manufacturing lens elements also made this desirable from a financial
standpoint. However, some of the rare glasses developed by Leitz are still not available to any other lens manufacturer. Instead Leitz has these glasses made exclusively for them by other companies like Schott, for instance. Still, the company looks
back with pride to the breakthroughs of the researchers and designers at the
laboratory.
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