[LargeFormat] Carl Zeiss Dagor 180mm f9

[Richard Knoppow]

----- Original Message ----- 
From: "Timothy Atherton" <tim at KairosPhoto.com>
To: "Richard Knoppow" <dickburk at ix.netcom.com>; "f32 Large 
Format Photography Mail List" <largeformat at f32.net>
Sent: Friday, September 30, 2005 9:51 PM
Subject: RE: [LargeFormat] Carl Zeiss Dagor 180mm f9


> Thanks Richard, tons of info.
>
> I borrowed one of a colleague for a while - it's a very 
> very nice lens (it
> was probably later rather than earlier and was in a COmpur 
> shutter). Someone
> was telling me it was a Protar III just renamed after 
> Zeiss bought Goerz,
> but I didn't think so.
>
> tim a

   There is a story to this. At about the same time that 
Emil von Hoegh was designing the Dagor Paul Rudolph was 
working on a similar lens. Rudolph's lens was also a double 
meniscus type with each half consisiting of three cemented 
elements but, in Rudolph's lens, the powers were in a 
different order. In the Dagor the outer elements are 
positive with the negative element sandwiched in between. 
Rudolph's version had the positive element in the center 
sandwiched between two negative elements. Actually, von 
Hoegh also covers this arrangement in his patent. von Hoegh 
got precidence on the patent but Rudolph also was able to 
get a patent on his version. Now, an interesting thing is 
that before he he took his design to  C.P.Goerz von Hoegh 
approached Zeiss. There is a cute story that von Hoegh was 
carrying a Christmas goose under his arm for his family and 
was not taken seriously by the Zeiss people. I think it is 
much more likely that he was rejected as a Zeiss designer 
because Rudolph knew von Hoegh had a rival design to his 
own. Zeiss did make some lenses of the Rudolph pattern as 
"Triple Protars" but never pushed the design much. The 
Schneider Angulon has the same arrangement of powers as the 
Rudolph design but uses oversize outter elements to get 
around mechanical vignetting. This is not a problem when von 
Hoegh's arrangement is used because it results in a shorter 
lens with less vignetting. There are other arrangements of 
three cemented elements in a double meniscus lens. The best 
known of these is the Voigtlander Kollinear. Curiously, 
there was another case of simultaneous and independant 
invention here, Steinheil also came up with the same design 
at about the same time. Steinheil called his lens the 
Orthostigmat. Patents were issued to both companies and 
actual lenses sometimes have the other's patent number on 
them. Evidently they cross-licensed their patents. None of 
the alternative forms has any advantage over the Dagor. The 
type has one fault: a large residue of zonal spherical 
aberration. This results in some softness when used wide 
open and some focus shift when stopped down. Actually, the 
blur due to the spherical is of a pleasant nature so Dagors 
are useful as mildly soft focus lenses when used near 
maximum aperture. Spherical aberration is proportional to 
the stop so Dagors in all variations become very sharp when 
stopped down about three stops. All double meniscus lenses 
are inherently wide angle lenses. Plain Dagors will cover 
(barely) 87 degrees at f/45.
   A single cell of a Dagor can be used alone but the 
correction for coma is dependant on symmetry so single cells 
must be used at quite small stops to be reasonably sharp, 
around f/36 being the maximum.
   The Zeiss index is a compendium of lens designs put 
together mainly by Willie Merte, one of the great Zeiss 
Designers. It includes all sorts of lenses made from around 
the late teens to the late 1930's. This internal document 
was captured after WW-2 and published. The lenses in it are 
included in the lens design survey program LensView, which I 
am fortunate enough to have by courtesy of its author. The 
Zeiss Index shows that Zeiss was evidently quite interested 
in the Dagor design and there are many variations of it 
included. Zeiss had a rival design which was introduced not 
long after the original Dagor. This is the Convertible 
Protar. As mentioned above while Dagor cells can be used 
alone for long focus lenses they are not corrected for coma. 
Coma is corrected by symmetry so a complete Dagor has very 
little but the individual cells have a lot. Paul Rudolph 
decided that by combining an "old glass" and "new glass" 
pair in a single cell (by cementing them, 4 cemented 
elements in each cell) he could obtain a true convertible 
where each cell was corrected for coma. Convertible Protar 
lenses were sold in sets of up to five cells of varying 
focal length along with a barrel and shutter. By using the 
cells alone or combining them a great variety of focal 
lengths could be obtained along with good performance. Used 
combined the Convertable Protar has no advantage over a 
combined Dagor and maybe less coverage, but individually the 
cells perform much better.
   Modern large format lenses and enlarging lenses are often 
of a type which traces its ancestery to the Dagor. This type 
is called the Plasmat. In effect, the Plasmat is a Dagor 
with the inner elements split off and air spaced. The 
advantage is that the additional degrees of freedom allow 
much better correction of the zonal spherical which plagues 
the cemented meniscus types. Plasmats are also capable of 
excellent correction for astigmatism and, like their 
prototypes, they are essentially wide angle lenses. The 
first Plasmat type was designed by Ernst Arbeit of Shultz 
and Billerbeck but the name comes from a lens designed by 
Paul Rudolph during his second career for Hugo Meyer. 
Another older lens of this general type is the Wide Angle 
Xpres (the regular Xpres is of a different type) made by 
Ross, and the Zeiss Orthometar of Merte. However, because of 
the flare from the eight glass air surfaces, this type of 
lens did not become popular with designers until afte good 
anti-reflection coatings became available after WW-2. 
Because air spacing can result in much improved correction 
and cheaper construction than is possible with all cemented 
lens designs such designs died out pretty quickly after 
coating became available.
   von Hoeg, by the way, was also responsible for the 
"dialyte" type lens. The first was his Celor but the type 
was perfected by designers such as Walther Szchokke, of 
Goerz, who designed the Dogmar and famous Apochromatic 
Artar, which are of this type.

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Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com