Re: [Leica] Glass issues

[chaz <cchoy@pacbell.net>]

Welcome back, long time no hear (rather read...)

chaz
- ----- Original Message -----
From: "Erwin Puts" <imxputs@ision.nl>
To: "LUG" <leica-users@mejac.palo-alto.ca.us>
Sent: Friday, July 27, 2001 11:21 PM
Subject: [Leica] Glass issues


> 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
>