Inspecting lens elements at Leica Camera AG in Wetzlar
Photo: M. Amling
One of the most important
substances needed to create photographic images is glass. It is used in the manufacture of photographic
lenses, digital sensors, camera viewfinders, SLR mirrors, filters, negative
carriers in enlargers, condensers in enlargers and projectors, slide mounts,
and more. As a matter of fact,
photography, as we know it today, would not be possible without glass.
The manufacture of glass
was already known to the Egyptians about 3000 BC and was further perfected by
the Phoenicians who are credited with the invention of the glass blowpipe. During Roman times, glass making was elevated
to an art form. Especially the
manufacture of highly decorated small glass vessels (fondi d’oro) became very
popular. This culminated in the 13th
century in Venice with the manufacture of highly artistic glass items. Yet even in the 16th century, glass, even
window glass, was considered to be a luxury item. It was also in the 16th century that the
first glass lenses appeared in the camera obscura, which was a major step in
the development of photography.
Roman glass vessels
According to the
dictionary, “glass is a hard, brittle, transparent or translucent, greenish
solid solution made by melting a mixture of silica (sand) and various
silicates” like soda, borates, phosphates and potash, with metallic oxides
added to give the product special properties.
Colorless glass is obtained by adding manganese, which neutralizes
traces of iron in the sand. Different
tints are obtained by adding various metallic oxides. For instance, gold and copper will result in
a red color, manganese violet, cobalt blue, iron and chromium green.
Depending on the
composition, some glass will melt at temperatures as low as 500° C (900° F),
others melt only at 1650° C (3180° F). After careful preparation and measurement,
the raw materials are mixed and undergo initial fusion before being subjected
to the full heat needed to turn the matter into glass, called
vitrification. In the past, melting was
done in clay pots heated in wood or coal-burning furnaces. Pots of fireclay,
holding from 0.5 to 1.5 metric tons of glass, are still used when relatively
small amounts of glass are needed for handworking. In modern glass plants, most
glass is melted in large tank furnaces, first introduced in 1872. They can hold more than 1000 metric tons of
glass and are heated by gas, oil, or electricity. The glass batch is fed
continuously into an opening (doghouse) at one end of the tank, and the melted,
refined, and conditioned glass is drawn out the other end. In long forehearths,
or holding chambers, the molten glass is brought to the correct working
temperature, and the mass is then delivered to the forming machines.
The above described
methods are useful for the manufacture of all but optical glass. The
manufacture of optical glass is a delicate and exacting operation. The raw
materials must be of the highest purity, and it takes great care to prevent
imperfections to be introduced. Small
air bubbles and residual unvitrified matter will cause distortions in a lens.
Striae, the streaks caused by incomplete chemical homogeneity in the glass,
will also cause serious distortion, and strains in glass caused by improper
annealing will further impair optical qualities.
Annealing is a process of
heat treatment by which glass is rendered less brittle and more resistant to
fracture. Annealing minimizes internal defects in the atomic structure of the
glass and leaves it free from internal stresses that might otherwise be present
because of prior processing steps.
Glass is annealed by
heating it to high temperatures and then cooling it slowly. Large masses of glass are cooled within the
heating furnace. Sheets are usually
annealed in a continuous-process furnace. They are carried on a moving table
through a long chamber in which the temperature is carefully graded from
initial heat just below the softening point to that of room temperature at the
end. The annealing time varies widely according to the thickness of the
individual piece. Window glass, for example,
requires several hours, plate glass, several days, and glass mirrors for
reflecting telescopes as well as certain glasses for photographic lenses
several months.
Optical glass was
originally melted in pots for prolonged periods, during which it was constantly
stirred by a refractory rod. After lengthy annealing, the glass was broken into
pieces. The best fragments were further reduced, reheated, and pressed into the
desired forms. In recent years a method has been adopted for the continuous
manufacture of glass in platinum-lined tanks, using platinum-lined stirrers in
the cylindrical end chambers (or homogenizers). This process produces greater
quantities of optical glass that are cheaper and superior to glass produced by
the earlier method.
Platinum crucible
Stirring rod
Pouring of molten glass
The principal
specifications for optical glasses are its light bending power, or refractive
index, for different wavelengths or colors of light and its dispersion, the
difference in refractive index for certain wavelengths. All of us have seen dispersion of light in
form of a rainbow from light shining through a prism and being separated in the
spectral colors. Dispersion is one of
the most difficult problems to deal with in designing photographic lenses. Dispersion will introduce chromatic
aberration which, when not controlled, will lead to poor sharpness and loss of
detail. The best optical glasses
available today display very high refractive indices with relatively low
dispersion. Within the last twenty years
some glass manufacturers have succeeded in producing glasses with a peculiar
property called anomalous partial dispersion, first pioneered by Leitz in the
early 1970s. Such glasses provide the
lens designer with new and powerful means to improve the color correction of
chromatic aberrations of camera lenses.
Prior to the availability of glasses with anomalous partial dispersion
some manufacturers used calcium fluorite crystals for the manufacture of lens
elements in telephoto lenses. Especially
Canon was making wide use of this. Other
manufacturers, however, do not regard them a suitable for photographic lenses
because they have very poor shape retention and because their refractive index
is only ne1.43. Because of this
relatively low refractive index, lens elements made of calcium fluorite require
very strong curvatures. This has the disadvantage
of increasing spherical aberration and causing substantially more surface
reflections.
Lens aberrations, all of which require different lens elements made from different types
of glasses to be corrected
The atlas of optical
glasses shows the refractive index and dispersion of many of today’s
glasses. The vertical axis shows the
refractive index for a wavelength of 546 nm while the horizontal axis shows the
Abbe number (for Professor E. Abbe of Zeiss), or the relative reciprocal
dispersion of the glass. This is a
comparison between the refraction of the glass for yellow/green light in the
center of the visible spectrum and the difference between its refraction for
blue/violet and deep red light. Please
note that these dispersion values are reciprocals: The higher the Abbe number, the less
dispersion. Sometimes optical engineers
need high dispersion numbers, sometimes low ones, to balance the
characteristics of different optical glasses in order to produce a high degree
of image correction.
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
The advances in glass
technology can easily be seen when comparing the refractive indices of glasses
over the years. The Leitz Elmar 50mm
f/3.5 from 1926 used SK15 glass with a refractive index of ne1.625, this
increased to ne1.694 with LaK9 glass in 1953 with the Summicron 50mm f/2. In the 1969 Summicron the refractive index
had grown to ne1.7479 with LaFN2 glass and the special 900403 glass developed
for the 1976 Noctilux 50mm f/1 increased the refractive index to ne1.9005
which, to my knowledge, is the highest refractive index for any glass used for
photographic production lenses. Leitz
uses a proprietary number to avoid divulging the make-up of this glass.
It was a long, arduous
journey to arrive at the modern glasses at the disposal of today’s lens
designers. Glasses with a relatively
high refractive index were available even prior to W.W.II. However, this could only be achieved by the
addition of thorium oxide to the glass.
Thorium oxide unfortunately is highly radioactive, and lens designers
began to search for a suitable replacement.
One of the problems encountered was that by replacing the thorium oxide
with a variety of other elements, the glass turned opaque. Kodak had some limited success during
W.W.II. They were able to make a thorium
oxide free glass with similar properties, but it was yellow in color. This glass, however, was used to make lenses
for some aerial cameras. Since these
were used exclusively with b&w films, the “built-in” yellow filtering
actually was found to be beneficial for this specific use.
At the Leitz glass
research laboratory similar experiments started in 1949. After lengthy experimentation they found that
lanthanum oxide offered the best chance for success as a replacement for
thorium oxide. They found that glasses could be made with a refractive index of
ne1.7. However, these glasses could not
be made in large quantities because they tended to crystallize. This makes it difficult to prevent crystal
grooves, or striae. Other substances
like zirconium oxide, yttrium oxide and tantalum oxide had to be added to
produce more stable high refractive glasses in large numbers. The resulting LaK9 glass resulted in the 50mm
f/2 Summicron in 1953, the first production lens free of thorium oxide. Leitz granted the German Schott company a
license to produce the LaK9 glass.
For lenses with a maximum
aperture of f/1.4 glasses with a refractive index close to ne1.8 are necessary.
However, these are difficult to
produce. Evaporation at the surface of
the molten glass, while the temperature is slowly lowered, can easily cause
striae. These can only be prevented by
constant stirring to produce consistent blending of the molten glass. This requires additional steps, so that one
oxide will prevent another from crystallizing.
These methods, for instance, have enabled Leica to use glasses with
refractive indices of ne1.80 and ne1.82 and a dispersion value of ny45. For glasses with refractive indices higher
than ne1.8 it is difficult to obtain enough chemicals with sufficient purity at
a reasonable cost. Subsequently such
glasses are very expensive.
The glass developed by
Leitz for the 50mm f/1 Noctilux is such a glass. It uses a much higher zirconium oxide
content, which allows for a refractive index of ne1.9005 and a dispersion value
of ny40. These glasses have a very high
melting point of 1600° C (2912° F). At
such high temperatures molecular migration of the material used for the
crucibles can contaminate the glass mixture.
To prevent this, crucibles of platinum or at least platinum lined
crucibles are used. The stirring tools
also have to be made either of platinum or must be platinum coated. In addition, the shape of the stirring tools
in relation to the viscosity of the molten glass is very important for the
homogenization of the glass. The
temperature of the melt in the crucible has to be lowered considerably before
it can be poured out. Then the poured
slabs have to be cooled at a carefully controlled rate, which may last days or
even months in order to prevent molecular tension.
Modern lens grinding machine at Leica Camera AG in Wetzlar.
For ultimate precision and accuracy, these machines grind one lens element at a time.
Photo: M. Amling
As mentioned before, most
lens manufacturers don’t consider calcium fluorite suitable for lens
production. Instead they prefer a true
glass with an amorphous structure. It
was discovered that fluorites remained stable in metaphosphate
suspensions. By incorporating several
fluorites, it is possible to optimize the proportions to avoid the creation of
striae during the cooling process. This
eventually led to the development of a true glass with anomalous partial
dispersion and a refractive index of greater than ne1.544. With the addition of titanium oxide Leitz was
able to produce a glass with an anomalous partial dispersion value of ny66.6 and
a refractive index of ne1.544. This
glass was used in the 180mm f/3.4 Apo Telyt R, originally developed for the
U.S. Navy, which to this day ranks as one of the best lenses ever produced for
a 35mm camera. Many of the large glass manufacturers have little interest in
the manufacture of such exotic glasses because optical glass is only of minor
importance to them. Construction
glasses, television tubes, bottles etc. are the main income generators. When these companies develop optical glasses,
they will do it only for those which will be in high demand. It is the exotic wishes of companies like
Leica, Zeiss, Schneider, Rodenstock, Nikon, Canon etc. that these glasses were
developed in recent years.
Leitz
used to operate their own glass research lab in Wetzlar. However, in the late 1980s this proved to be
economically no longer feasible and the lab closed. Since then Leica has been buying optical
glasses on the open market from manufacturers like Schott and Hoya. These companies, prinmarily Schott, also make
the proprietary glasses for Leica.
However, that does not mean that these glasses are available to other
manufacturers. These are exclusively
made for Leica and their chemical composition remains a closely guarded secret.
The next time we complain
about the high prices of some of the top lenses on the market, we should
remember the difficulties encountered in just the manufacture of the
glass. Add to that the great care which
has to go into the production of the individual glass elements and the
mechanical components of the lens, and the prices somehow don’t seem to be all
that high after all.
________________________________________________________________________________________
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