Re: [Leica] Glass issues

[Dan Cardish <dcardish@sympatico.ca>]

But Erwin, surely quality control of the glass production is important?
It's one thing to look up and order a certain glass from a specifications
catalogue, but if the manufacturer is unable to gaurantee that the
delivered glass is exaclty what you ordered, then those specifications
don't mean that much.

dan c.

At 11:21 PM 27-07-01 -0700, Erwin Puts wrote:
>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
>
>