[rollei_list] Re: Tessars
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 2 Jun 2007 15:33:00 -0700
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, May 31, 2007 7:12 PM
Subject: [rollei_list] Re: Tessars
At 09:58 PM 5/31/2007, Richard Knoppow wrote:
> How did you determine the age of the two lenses?
> Zeiss held a great many patents for Tessar type
lenses.
>Its nearly impossible to determine which are for
production
>lenses. Note that the Kodak Ektar is a Tessar type lens
and
>generally outperforms Zeiss lenses of the same era, so its
>possible to make a better Tessar.
Richard
On the first point, I would suspect that the ages
were determined by serial numbers. If Peter
would share the body and lens serial numbers with
us, it would make our task a bit easier.
On the second point, I splutter in
protest. Kodak made many a fine Tessar-derived
Ektar but to make a blanket statement of the sort
you made is simply to make yourself appear
foolish. In an era of hand-assembled lenses, it
is undoubtedly true that a given Kodak Ektar on a
given day could outperform a given Zeiss Tessar,
but to suggest that the Kodak lenses inevitably
outperformed the incapable lenses produced by
those hapless jerks at Zeiss seems to be
stretching the case quite a bit more than the record
permits.
Marc
My statements are based on experience with enough of
both lenses to see a consistent difference. I am speaking of
f/4.5 and f/6.3 Tessars and Ektars. Specifically, the
135mm, f/4.5 Tessar and 127mm f/4.7 Ektar as used on Speed
Graphic cameras. Both are excellent but the Ektar will show
slightly better resolution. This can be seen in the aerial
image as a slight increase in contrast of very fine detail
like textures. I've examined both coated and uncoated Ektars
but only uncoated Tessars. The Tessar has better suppression
of ghost images. The Tessar has a definite ghost image of
very bright objects anywhere in the field. This can be seen
by looking at a bare light bulb on the ground glass. The
Tessar surfaces seem to have been calculated so as to
eliminate this ghost.
Neither lens will be sharp to the edges wide open
because the Tessar design suffers from something called
oblique spherical aberration. This is inherent in the type.
The effect is similar to coma, that is, an unsymmetrical
blure spot off axis. Both lenses appear to be free of it at
about f/8 and are very sharp at around f/11.
From the patent information and notes in the Zeiss
Index, both of which are in LensView, Zeiss continued to
experiment with the Tessar design from its invention until
the break up of the company. There are designs using one or
more aspherical surfaces for instance.
Kodak also experimented with the design. In particular
George Aklin held a patent on a form of Tessar with reversed
powers in the rear component. He claimed this was
advantageous where high index glass was used. The Kodak
Anastigmat Special, a front element focusing lens, is of
this type.
According to Brian Caldwell, a well known lens designer
and author of LensView, many of the Zeiss Tessar designs are
so good that running them through a computer optimization
program could not improve them.
Its quite possible that post-war Tessars were even
better. It would take careful testing on an optical bench to
really determine the relative performance of Zeiss vs: Kodak
lenses. I don't have access to this kind of equipment.
BTW, as far as I can find Kodak did not use rare earth
glass in its Tessar type Ektars. However it did use such
glass in other types of lenses sold under this trade name. I
am not quite sure now but think the first lens to use
Lanthanum glass was an f/1.9 lens made for the Ektra camera.
The first lens to bear the name Ektar was a six element
Opic/Biotar type, 45mm, f/2, made for the Kodak Vigilant in
1937.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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