[LargeFormat] Double Protar Lens

[Richard Knoppow]

>
> Richard,
>
> Thank you very much indeed for the additional information,
> particularly the difference in effective apertures
depending on
> mounting position of the individual cells and the optimum
stop to use.
>
> The coverage is really small using the single cells. Is
there a sharp
> illumination cut off or are we talking soft at the edges?
>
> Do you know roughly when the Protar was redesigned? All
those I've
> seen have an 1895 patent date but there again that's a
pretty small
> sample.
>
> There have been a lot of rather negative comments on the
convertible
> Symmars used in converted mode.  Low contrast and probably
some
> softness due to aberrations. Yep, it's amazing that the
Protar design
> can not be improved by much even using the number
crunching of
> contemporary technology. I bet the lens designers had a
lot of late
> nights and piles of discarded hand ground lens
elements..... Wasn't
> it Emil von Hoegh who developed the maths to push forwards
the
> boundaries of lens design? Maybe it was a Goerz
relationship but he
> seemed to have been an independent thinker and willing to
share his
> techniques.
>
> Cheers,
>          Clive
>
>
   I don't think this was Emile von Hoegh. He was a very
good designer but I don't think he was responsible for any
of the mathematics for lens calculating. I will have a look
at Kingslake again to make sure.
   He came to Goerz with the Dagor after trying Zeiss. While
there are some cute stories of why Zeiss turned him down
(such as the one about his carrying a goose for dinner) the
real reason may be that Paul Rudolf, the designer of the
Protar and Tessar, was working on a very similar design at
the time. Rudolph's lens had the order of powers reversed.
Actually, von Hoegh's patent covered this design as well as
the standard Dagor design. von Hoegh also came up with the
four element air spaced type known as a Dialyte. He derived
this from the Dagor by splitting the cemented interfaces and
replacing the center element with an "air lens". The
resulting lens was sold as the Celor. Later the Dogmar,
Apochromatic Artar, and many other lenses were based on this
design.
  Compared to the Dagor the Dialyte has much narrower
coverage but little zonal spherical aberration. They are
widely used as process lenses and enlarging lenses.
  The Protar was designed by Paul Rudolph of Zeiss. His
original Protar was a four element lens consisting of two
cemented components. The front was made of "old" glass, the
rear from the new high index "Jena" glasses developed by
Abbe and Schott. This was thought to be the first anastigmat
lens although there seems to have been a predecessor. The
Protar was originally called the "Anastigmat" but the name
became generic too quickly for Zeiss to protect. They
changed the name to Protar (meaning the first). Several
Protar lenses were designed and built by Zeiss. The original
four element design was made in several versions with speeds
from around f/5 and "normal" field coverage to an extra wide
angle lens with speed of f/18. The latter was made by both
Zeiss and Baucch & Lomb until the late 1940's. I think it is
still in the B&L catalogue as late as the 1950's.
  The Convertible Protar was designed by Paul Rudolph as an
improvement on the Dagor. While the Dagor is patended and
was sold as a convertible, it really isn't. The individual
cells are not corrected for coma so sharpness is not good
until the lens is stopped down to around f/45. Coma is
automatically corrected in symmetrical lenses, so the
combined Dagor does not suffer with this problem.
  Rudolph added an element and corrected the individual
cells for coma. Their peformance as individual cells is
substantially superior to the Dagor but the performance of
the combined lens is similar.
  Zeiss did build a lens of Rudolph's alternative Dagor
design under the name Triple Protar Series VI.
  Rudolph's original patent for the Series VII protar is
dated1894 is and USP 523,398. In 1912 a redesigned lens was
patented under USP 1,021,337  I give the US patents rather
than the German ones because they are accessible on the web.
Any US patent ever issued can be gotten from the U.S.patent
and Trade-Mark Office site at http://www.uspto.gov  Patents
older than 1975 can be searched by patent number only. They
can be downloaded in the form of Class-4 FAX tiff files.
These can be read by several programs. The best plug in for
browsers is Alternatiff, freeware. Do a Google search to
find it.
  BTW, I was wrong about the "C" set, is had only three
cells. There was a "D" set with four. The B set also had
hree cells, for 4x5 cameras. The C set three cells for 5x7,
and the D set four cells for 7x9.
  The coverage of the invidual cells is limited partly by
vignetting and partly by aberrations. The circle of
illumination is not large. Maximum coverage is on the order
of 45 or 47 degrees for good quality although lenses will
cover a diagonal about equal to the focal length when
considerably stopped down. They were meant to give long
focus capability to the lens for portrait work, etc.
  The problem with the Convertible Symmar is that it was
compromised to make the individual cell performance better.
Since most users were intested in best overall quality
rather than in the convertible capability the lens was
redesigned later as a fixed focal length lens. Despite this
you can always use one cell at a time for longer focal
length. The quality is not bad but its a magtter of
indivudual judgement whether its good enough.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com