[rollei_list] Re: Planar vs. Planar
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 3 Mar 2010 21:43:02 -0800
----- Original Message -----
From: "Laurence Cuffe" <cuffe@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, March 03, 2010 3:06 PM
Subject: [rollei_list] Re: Planar vs. Planar
Thanks Richard, thats pretty much what I thought.
One other comment on the planar lens design:
My understanding is that the original Planar got a poor
press because of internal reflections, which finally came
under control when elements could be given anti
reflective coatings. I think Kingslake is my source for
this.
All the best
Laurence Cuffe
The original Planar was manufactured and sold by Zeiss
for many years. It was a successful design. The lens has
eight glass-air surfaces, lenses like the Tessar and Cooke
Triplet have six. The internal reflections leading to flare
go up geometrically with the number of surfaces and about
eight is all that work well without coatings. Even then the
effective speed of the lens is reduced significantly.
Nonetheless a great many uncoated lenses with eight
glass-air surfaces were made and sold over the years. The
famous Zeiss Biotar is one and the Speed Panchro of H.W.Lee,
which nearly universally used for professional motion
pictures is another. With careful design such lenses can be
made to have minimum ghost images which is probably more
important than overall flare.
Lee was the first to modify the Planar design to
increase its speed from f/4.5 to f/2. This was the Opic of
TT&H. The Opic was just another lens in the TT&H catalogue
so was did not have much influence. The Biotar however, was
sold for some popular cameras of the time and became quite
successful. The Speed Panchro is essentially another version
of the Opic. The Leica f/2 lenses were all based on the
Planar/Opic as was the Schneider Xenon. The original Planar
fell by the wayside because there were other lenses of equal
speed that were less espensive and had less flare.
Nonetheless it was a successful lens.
Practical lens coatings revolutionized lens design;
before coatings designers did whatever they could to
minimise the number of glass-air surfaces so one finds
designs like the f/1.5 Sonar, a seven element lens with only
six glass-air surfaces because most of the interfaces were
cemented. The famous Zeiss Convertible Protar is another
example. Each cell has four elements but they are cemented
so there are only two glass-air surfaces in each half. The
flare level of lenses like the Convertible Protar and the
Dagor is very low even without coatings and such lenses were
popular for color work partly because of their good contrast
and color purity.
But- cemented surfaces are expensive to make and limit
the designer because he/she can not curve the surfaces
independantly.
Working coatings were developed very early but they
were not practical for production lenses. Smakula, of Zeiss,
worked on vacuum deposition of metallic oxide coatings as
early as 1935 but Zeiss does not appear to have applied them
to production lenses. Perhaps because the early methods
produced coatings which were too soft to withstand normal
cleaning. Others experimented with chemical coatings. Here
again, the results were successful in terms of reducing
surface reflections but the coatings were not rugged enough
to be practical.
Eastman Kodak began using a soft coating on the
internal surfaces of some of its premium lenses about the
late 1930's. Among these were the lenses for the ill-stared
Ektra camera, the Eastman Ektar series (which later, with
the addition of hard coatings on all surfaces, became the
Kodak Commercial Ektar) and a few others. These coatings
were on protected internal surfaces only.
The technique of hard coating was developed in the US
during WW-2 (sorry Marc, this is well documented). The
technique was based on the discovery that if the coatings
were baked in the vacuum chamber in vacuuo the coatings
changed form and became as hard as the glass. The project
was done by a government research group which included
people from Eastman Kodak, Bausch & Lomb, and I think Perkin
Elmer and others who were engaged in production of optical
devices for the military. Following the war hard coatings
began to be applied to consumer lenses within a very short
time. Kodak and Wollensak were among the first to announce
coated lenses, both c.1946. Some of the smaller companies
were slow to undertake coating, for instance
C.P.Goerz-American never routinely coated their lenses
although by the mid fifties they were offering most lenses
with coatings. Goerz made mostly low flare lenses, the Dagor
being their leader, plus they made process lenses, the
Apochromatic Artar being their main one. Process lenses are
not affected by flare since it can be corrected for by a
small adjustment in exposure when making half-tone plates.
There were also a few companies advertising after-market
coating. I am not sure how successful this was since coating
a lens requires it to be subjected to enough heat to require
recementing of lenses cemented with Canada Balsam at least.
In any case, by the late 1940's coatings were being
applied routinely by all of the larger optical manufacturers
both here and in Germany. Designers began to take advantage
of the ability to used air-spaced elements and some lens
types which had been dormant began to be very popular. The
Planar/Opic/Biotar was one and the Plasmat was another. The
Plasmat is a six element in two groups lens originally
derived from the Dagor by air spacing one of the elements.
While the Planar has a single lens at the outside and
cemented elements inside the Plasmat is the reversel with
the cemented groups on the outside and single elements in
the air space between the front and rear elements and the
iris. Like the Dagor the Plasmat is essentially a wide angle
lens but has very much better correction for spherical
aberration than the Dagor and can be unusually well
corrected for astigmatism. These lenses have become very
popular for view cameras and similar applications. Examples
are The Rodenstock Sironar and Schneider f/5.6 Symmar (there
was an earlier Symmar at f/6.8 which was a clone of the
Dagor). Both companies also use the Plasmat type for their
enlarging lenses. The Plasmat simply had too much flare
without coatings and was not very popular. Unlike the lenses
based on the Planar, which were unusually fast, Plasmats had
to compete with the Dagor, Convertible Protar and similar
lenses with much less flare. While it was better in other
ways it had too much competition to sell well. The main
competition to the Opic/Biotar was the Zeiss Sonar which had
lower flare but had some other problems all of which are
inherent in the Triplet on which its based. Also Zeiss made
the Sonar mainly for its own Contax camera.
Modern coating techniques allow the use of multiple
layer coatings. These have the advantage of working over a
broad band of colors. A simple single layer coating is
effective at a single wavelength although in practice they
have enough bandwidth to reduce flare over the visible
range. A properly designed and executed multiple coating can
reduce surface reflection to almost nothing over a wide band
of colors depending on how many layers can be tollerated
from a cost standpoint.
Note that coatings can also _increase_ reflection and
such coatings are commonly used on mirrors. For example, an
aluminum mirror has wider bandwidth than a silver one but
has lower reflectance in the visible range. By applying a
coating the reflectance of the aluminum can be brought up to
an even high level than the silver and the combination still
retain the mechanical toughness and resistance to oxidation
of the aluminum. Many other substances have become practical
for mirror use through the same thin-film vacuum coating
technology used for lens coating. Plus it is used for the
production of a wide range of electronic devices.
For more including a downloadable book on coating
history, see the Society of Vacuum Coater's web site:
www.svc.org
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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