[rollei_list] Re: Whiteface T with Xenar question
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 19 Mar 2009 10:11:50 -0700
----- Original Message -----
From: "CarlosMFreaza" <cmfreaza@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, March 18, 2009 3:35 PM
Subject: [rollei_list] Re: Whiteface T with Xenar question
Hi Richard:
The Rolleiflex T with Xenar lens is
referred by
Prochnow in the Rollei Report 2 _second and reviewed 2001
edition_ ,
it's the Prochnow Register 185/2, PR 185/2. The T with Xenar
3.5/75
lens was manufactured from the beginning of 1973 to August
1976, 3000
units from serial numbers T2.310.000 to T2.320.449.
Rollei specs for Xenar and Tessar lens were identical and
then the T
Xenar is at least as good as the T Tessar in theory.-
Carlos
Having read most of this thread I need to add a couple
of things about lenses and lens design. Tessar refers to an
arrangement of lens elements originated by Dr. Paul Rudolph
at Zeiss not long after the then revolutionary optical glass
types, sometimes called Jena glass, were developed by
Schott. The previous types of glass did not have suitable
combinations of index of refraction and dispersion (change
in the index with color) to allow the making of anastigmatic
lenses (there is a qualification here but for the moment its
irrelevant). Rudolph began to use the new glass types to
design lenses which could be free of astigmatism and
simultaneously have a flat field and be color corrected.
After a few first steps (the original Protar was the first)
he came up with the Tessar design. The Tessar is a fairly
simple lens consisting of two cemented elements and two air
spaced elements. While this generic description fits some
other types the order of power and some other factors are
different so the Tessar remains a specific generic type. The
Schneider Xenar is a Tessar _type_ lens. Both the original
Rudolph design and the Xenar design have been changed many
times since they were first produced. The main changes in
the case of the Tessar were to increase the speed. Rudolph's
original lens was about f/8 but other designers at Zeiss,
notably Ernst Wandersleb, were able to increase the speed to
about f/2.8. The Xenar was first produced after the original
Zeiss patent on the Tessar had expired so Schneider was able
to take advantage of some of the improvements.
Before WW-2 Schneider was known mostly for cheap lenses
but after the war it began to produce very good lenses and
was ahead of Zeiss mostly because Zeiss was in the East zone
and Schneider was in the West. In any case, post-war
Schneider lenses began to be known for excellent design and
quality of manufacture.
For the last at least thirty years much lens design has
been done by computer aided methods. Designing a lens is
heavily mathematical involving extensive computation. Until
the advent of computers the evaluation of a proposed design
had to done by making up a sample lens and measuring it.
Then, if the sample was reasonably good it could be adjusted
to improve it. Modern lens design programs can completely
evaluate a design in a few seconds and make calculations
that would be completely impractical if done by hand.
Changes in design are also made by computer. Most lens
design programs include automatic optimization. The program
still needs human guidance but can find small changes that
make a significant difference. The programs can also show
the difference in performance produced by a change in glass
types and calculate the sensitivity of the design to
manufacturing tolerances. One result of this is that there
is getting to be less and less difference in lenses of
similar types made by different manufacturers. Quality
control in manufacture is still an issue but is probably not
significant among the large manufacturers.
The computer program can do only what the operator
tells it to do. If the specs given to it are the same or
similar the results will be about the same but the specs are
not always the same. For instance, the cost of optical glass
varies greatly from one type to another and there are other
considerations such as ease of grinding and long term
stability. A difference in the limits of the glass which can
be used will affect the final performance.
Some years ago environmental protection regulations
forbid, or made expensive, the use of certain materials in
optical glass. For instance, arsenic and lead were long used
as clarifiers in glass of all types and can not be included
now. Because this eliminated many glass types with superior
optical characteristics many lenses had to be redesigned to
make use of the glass types which remained available. This
is the real reason for much of the redesign which has taken
place since about the 1970s. In some cases the lens
performance was improved but in many others the redesign was
simply to try to maintain the quality in the absence of the
glass types formerly used.
In about the mid- 1930s the United States Bureau of
standards came up with a new series of optical glasses using
"rare earth" instead of silicon. These glasses had
remarkable qualities which allowed the significant
improvement of many lens types. Commercial production and
development of these glasses was undertaken by Eastman Kodak
and soon the new glasses were applied to a number of
commercially made lenses. Among the rare earths used was
Lanthanum. Lanthanum allows a glass with a much smaller
dispersion compared to its average index of refraction and
also a very high index. The index is important because the
higher the index the less curved the lens needs to be to
have the power it needs. Since some types of aberrations are
proportional to the curvature these lenses can have lower
aberrations than those using strictly either "old" glass or
Jena (barium) glass. Not all lenses can be improved much so
the mere presence of a rare-earth glass does not guarantee
that a lens has high performance.
According to a friend who is an actual lens designer
many of the old Zeiss designs can not be much improved by
modern lens optimization programs, that is, Zeiss did a very
careful job of design. Also he pointed out that a number of
common lenses were not improved much by the use of modern
glass.
Every generic lens type has some inherent problem. In
the case of the Tessar it is oblique spherical aberration.
This is an aberration which looks similar to coma but comes
from a different source in the lens. It results in smearing
of the image away from the center and is reduced by stopping
down. This aberration limits the practical speed of the
Tessar type lens.
BTW, some of the rare-earth glasses were quite
radio-active and the activity was such as to be quite long
lived. In particular thorium glass is quite radio-active.
Thorium was used as an ingredient in several
high-performance glasses before being banned. Some sources
state that it was "an impurity" but that is not true, it was
present deliberately because it produced desirable glass
characteristics. Notable lenses containing thorium glass are
the Aero-Ektar series. Many of these lenses have stained
(browned) elements because of the long exposure to the
radiation from the thorium-containing elements. The
designers probably knew that the glass would not remain
clear long but the glass was necessary to get the
performance needed and the lenses were not expected to have
a working life of more than a few years.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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