[Leica] Glass issues

[Erwin Puts <imxputs@ision.nl>]

The current discussion about the quality of filters and the differences in
glass manufacturer is to a large extent based on partial information, and in
danger of being wrongly interpreted.
Some basics.
Every optical glass is defined by two properties: index of refraction and
dispersion (referred to as the Abbe number).
If you would look in a glass catalogue of a major manufacturer (as example
Schott) you will see a glass map, which has on the horizontal dimansion the
Abbe number and on the vertical axis the Refraction value. All types of
glass can be located within this coordinate system. Glass wih a specific
combination of Abbe number and Refractive index can be identified by a name
or a number: for some glass Schott calls it BK7, Corning calls it B-16-64,
Hoya calls it BSC-7, O'Hara calls it BSL-7 and the official designation is "
517624".   
Whatever you name it and who will produce it, all relevant characteristics
are identical. There are of course differences in glass composition, thermal
processing, homogeneity and stain resistance etc.
But the optical properties are identical.
If a filter company needs glass with some specifications, they will specify
the requirements and select a glass. It does not matter at all whether this
glass is provided by Hoya, Schott (in Germany or Malaisia), Corning or Ohara
or Minolta or you name it. Sometimes the Schott version of the glass is
better sometimes Ohara or Hoya.

While most companies manufacture glass that has been created by Schott,
there are also many glass types by Hoya or others that have no equivalent in
the Schott catalogue.

The whole discussion about the quality of glass being related to a
manufacturer is wrong. You have to look at the specifications and then
select a manufacturer that is closest to these specs.
While Schott is still the reference, there are many glass types from others
who surpass the Schott glass.
The idea that a filter made from Schott glass must be superior to one made
from Hoya glass is untenable.

In fact many lens desigersoften  prefer Hoya glass and not Schott glass even
when the numbers are identical, because the characteristics of the Hoya
glass are superior to the Schott glass for the application.

Remember too that glass is made in four categories of quality. So Schott
glass of class 2 is not as good as Hoya glass category four.

 
You have to study the glass catalogues and the characteristics to be sure of
what a glass does. There is not a one to one correspondence between
manufacturer-glass type-quality.


Erwin