[rollei_list] CZ article about the Planar lens
- From: CarlosMFreaza <cmfreaza@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 21 Jul 2011 20:21:22 -0300
Last Carl Zeiss Camera Lens News magazine (Nº 40) contains an article
about the Planar lens written by Dr H H Nasse from CZ Camera Lens
Division, I extracted some paragraphs about the lens origin and those
ones linked to the Rollei TLR cameras, the article is larger and has
diagrams and documents reproduction (see URL below):
1) The first Planar lens was registered for patent by its
inventor Paul Rudolph at Carl Zeiss in Jena, Germany at the end of
1896 and was added to the Carl Zeiss product range in various focal
lengths as early as 1897. In other words, it is even a few years
older than the Tessar lens. The lens developed by Rudolph had a strict
symmetrical design comprising six lens elements assembled in four
groups, featuring a pair of meniscus shaped cemented elements in front
and behind the aperture. (In optics, meniscus refers to a lens on
which the centers of curvature of both surfaces are on the same side).
2) Despite all of these favorable features, the Planar lens enjoyed
only marginal success in the beginning. Although the older double
anastigmatic lenses (later: Protar) were not
quite as good, they were slightly more versatile, because the front
and rear lens halves could be used alone, therefore allowing three
focal lengths with a single lens. This was not possible with the
Planar.
In particular, the Planar lens was considerably more sensitive to
bright light sources in the image due to its eight glass-to-air
surfaces and unfavorable curvature. Antireflective lens coatings had
not yet been invented. This meant that unwanted optical paths of
several reflections in the lens created ghosts and glare in the image,
because each glass-to-air surface reflected around 4% of the incident
light.
3) It was not until the 1920s that optic designers resumed efforts to
advance the double Gauss lens. Their primary objective was to increase
its speed. In 1927, for example, Willy Merté at Carl Zeiss in Jena
designed an entire series of lenses for 35mm cameras and 16mm movie
film with a maximum aperture of f/2 and f/1.4.
These new designs entered the market under the name Biotar. Its design
was very similar to the original Planar lens, but it abandoned the
strict symmetry approach for the radii of curvature of the surfaces
and the refractive indices of the glass materials and therefore
achieved additional correction parameters.
4) However, after Alexander Smakula invented antireflective lens
coatings at Carl Zeiss in Jena in 1935 and their broad usage after the
end of World War II, it was possible to tap into the full potential of
the double Gauss lens without any negative "side effects", and
outstanding lenses were created for a wide range of applications.
The Gauss models designed at Carl Zeiss Oberkochen never used the
Biotar name, but kept the older Planar brand name for historical and
political reasons. As a result of World War II, the Carl Zeiss company
was divided into an eastern part (Jena) and a western part
(Oberkochen). The two companies manufactured similar products and were
embroiled in legal conflicts about the use of trademarks that spanned
several years. And since Carl Zeiss Jena lodged a claim to use the
brand name Biotar, Oberkochen used the name Planar. Both lens names
can be found on the twin-lens Rolleiflex cameras made in the early
1950s, as lenses for these cameras were delivered from the east and
the west back then.
At Carl Zeiss Oberkochen, Planar was also a 5-element Gauss model,
which, thanks to advancements in glass technology, was invented to
simplify the design without compromising performance. In Jena, this
type of lens was called Biometar.
5) This 5-element Planar model enjoyed notable success in various
camera formats, from the wide angle 3.5/35 for the Contax rangefinder
camera to the 3.5/135 for the 9x12 large format field camera. In
particular, a number of exceptional pictures were taken with these
optics during the heyday of the twin-lens TLR camera. It is also a
wonderful example demonstrating that image quality is not merely
produced by the number of lens elements: in fact, there was also a
7-element Planar model for the 6x6 format, yet it was not better. In
fact, more optical efforts are needed, as the design conditions for a
camera with a swing-up mirror are slightly more unfavorable than those
for a rangefinder or large format view camera that permit shorter
back focal lengths, i.e., the distances of the rear lens element from
the image plane.
The entire PDF article is here:
http://tinyurl.com/3knmf8m
Carlos
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