[rollei_list] Re: Question re. Planar vs. Tessar
- From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sat, 2 Apr 2005 23:26:53 -0800 (GMT-08:00)
-----Original Message-----
From: Ardeshir Mehta <ardeshir@xxxxxxxxxxxx>
Sent: Apr 2, 2005 10:02 PM
To: rollei_list@xxxxxxxxxxxxx
Subject: [rollei_list] Question re. Planar vs. Tessar
Hi all,
I just read at <http://johnlind.tripod.com/zi/zeissikontext.html> the=20
following:
[QUOTE]
In 1896 Dr. Rudolf created the Planar, derivatives of which live on=20
today most notably in the legendary Rollei TLR. The original symmetric=20
Planar suffered from flare (lens coatings were not invented yet), but=20
was free from spherical aberration and astigmatism. Reducing the flare=20
issues with the symmetrical Planar resulted in the design of the=20
assymetrical Tessar in 1902. The Tessar quickly became known as "The=20
Eagle Eye" because of its extremely high resolution and excellent=20
contrast with very low distortion. The Tessar is a simple lens with=20
four elements in three groups and yet remains one of the finest lens=20
formulations of the 20th Century.
[END QUOTE]
So I gather that the Tessar is supposed to be an IMPROVEMENT over the=20
Planar. Why, then, is the Planar generally preferred over the Tessar=20
among users of the 6x6 format, whether SLR or TLR? Any ideas? Richard=20
... ?
Ardeshir <http://homepage.mac.com/ardeshir>
Rudolph derived the Planar from the double lens that C.F.Gauss proposed =
as a telescope objective. Rudolph compounded the inner element and made the=
lens symmetrical. Its problem was not particularly flare although 8 unocoa=
ted glass air surfaces do have relatively high flare. The symmetrical versi=
on of the lens was optimised for unity magnification and had some problems =
for distant objects. In the early 1920's Horace W. Lee, of Taylor, Taylor &=
Hobson, designed the Opic, a lens based on the Planar but with some power =
shifted from the front to the rear cell. This optimised the lens for distan=
t objects and overcame the problems the original Planar had. Several years =
later Zeiss produced a similar lens under the name Biotar. The Biotar is a =
much better known lens than the Opic so its name is often used as the gener=
ic name for the type. The Opic form of the Planar has proven to be an enorm=
ously powerful design and thousands of lens designs are based on it.=20
The performance of a Planar/Opic/Biotar is superior to the Tessar. Tessa=
rs suffer from oblique spherical aberration, which is difficult to correct,=
so they are not suitable for lenses faster than about f/2.8 and actually a=
re best at around f/6.3. The six element Biotar is good to about f/2 and th=
e addition of an additional element will extend this to f/1.4 or even faste=
r. The Biotar is a more expensive lens to make than a Tessar because it has=
more elements and has a second cemented surface. Cemented surfaces are exp=
ensive to make because the two surfaces that are to be mated must be indivi=
dually ground to match and there is handwork in the extra centering operati=
on and the cementing operation. Before good lens coatings were available de=
signers tried to avoid glass air surfaces to reduce flare. This resulted in=
designs like the Sonnar, the Convertible Protar, and even the Dagor, where=
cemented surfaces were used in preference to glass air ones. Once good ant=
i-reflection coatings became available the greater freedom given by glass a=
ir surfaces was taken advantage of by designers and most of the multiple ce=
mented surface lens designs were not longer used.=20
The Tessar is remarkable because it is a relatively simple lens which is=
nonetheless capable of excellent performance within its speed and coverage=
limits. The Zeiss Index shows that Zeiss continually fiddled with the Tess=
ar design trying to improve it. The Index shows several Tessar types dating=
from probably before 1930 and maybe from even earlier, with aspherical sur=
faces. A couple of these have virtually no spherical aberration. Such surfa=
ces were prohibitively expensive to make at the time so I doubt if many wer=
e found in commercial lenses.=20
The Sonnar uses blocks of low index glass in place of airspaces. This a=
llows some of the advantages of glass-air surfaces with the low flare of a =
cemented surface. Essentially, the Sonnar is a derivative of the Taylor Tri=
plet. and has its problems with spherical aberration. The Biotar can be mad=
e nearly free of spherical so it has an advantage if flare can be controlle=
d.=20
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
Los Angeles, CA, USA
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