[rollei_list] Re: Whiteface T with Xenar question
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 21 Mar 2009 16:22:16 -0700
----- Original Message -----
From: Sanders McNew
To: rollei_list@xxxxxxxxxxxxx
Sent: Saturday, March 21, 2009 7:40 AM
Subject: [rollei_list] Re: Whiteface T with Xenar question
I am always happy to stir the pot on an obscure
Rolleiflex question. And I'm feeling my oats these
days: Melanie is expecting our first child, a boy!
So life is good.
(BTW, Melanie sends her regards to Richard and
Slobodan.)
Back on subject ...
Peter, when you opine that they are "probably"
identical, that can mean only two thing: Either
(1) Schneider made conforming changes to the
Xenar to match the reformulated lanthanum
Tessar, or else
(2) Zeiss's reformulation of the Tessar did not
improve the lens.
Which was it?
Sanders
Well, my love to Melanie, who is quite attention
getting in person!
Its very difficult to know how to answer questions like
these unless the actual prescriptions for the lenses are
available and then one has only the calculated performance.
To obtain actual performance on any lens its necessary to
run a number of well controlled measurement on it using
precision optical instrumentation. The prescription is the
"formula" for the lens. It consists of specifications for
the curvature of each surface, the thickness of each element
and each air space, the distance of the design object plane
(at what distance the lens design is optimized), the
_complete_ specifications for the glass from each element is
made. The latter is available from the manufacturer's
catalogue if the actual maker and glass type are given.
There are other parameters in an actual production
specification such as the tolerances of of performance
allowable. I give these details, as I did in my previous
post, to give some idea of how complex optical design and
production is and how inadequate most of the common informal
methods of evaluating lens performance are. In addition to
this we have the variations introduced by coating. While
coating does not affect the resolution or aberrations of a
lens directly the effect on image contrast can affect the
usable resolution since resolution is ultimately a contrast
measurement.
Now having said all that, the change in a design might
be for improving the lens because, for instance, an error is
suspected in the original design, or because some new glass
type can improve the performance or lower the cost for
similar performance. Also, many older designs had to be
redesigned when stronger envirionmental regulations made it
more difficult or impossible to include certain traditional
chemicals in the glass. This changed the glass constants and
required a redesign to accomodate what ever glass had to be
used. Such a redesign may or may not improve a lens, the
primary object is to duplicate, if possible the former
design. There are many examples but one is that Paul
Rudolph's original Tessar can not be duplicated with glass
types available today.
For the most part re-calculation of a generic type is
done using a computer design program. There are many
available. The full programs are quite expensive but there
are demonstrator versions available free which will give you
some idea of how they work. Usually, these are limited as to
the number of surfaces one can enter and may have other
limits. There is actually one complete program available
free but its a command line program and beyond me.
So the direct answer to your question is that no one
outside of the factories can be sure if a lens was actually
redesigned or what the object or results of the change were.
In addition, records from Zeiss show that the company
was constantly experimenting with variations of their
lenses, at least at the period that the _Zeiss Index_ covers
(probably late 1800s to perhaps the twenties). Even where
one has patent and other information on lenses it very
difficult or impossible to tell which designs were actually
used in production and are in cameras (with some
exceptions). As far as the effect of Lanthanum glass on a
Tessar type lens, my lens designer friend set up a number of
Zeiss designs in a modern lens design program and allowed it
to do whatever it could to optimise them including using
modern low dispersion and rare-earth glasses. The results
were that not much improvement was gotten from the glass
change.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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