[LargeFormat] plastigmat lens

[Richard Knoppow]

At 05:27 AM 08/22/2002 -0700, you wrote:
>the patent that Richard mentions discusses the possibilities of 4 
>element lenses, (no mention of lens names, as that would be limiting) 
>with some positive and some negative elements and their positions.
>
> From this same patent Ed Bausch brought out the Unar.  Their was a 
>codicil amending the patent when it was discovered that one of the their 
>combinations turned out to be the Protar. The codicil acknowledged that 
>Zeiss had the patent on that combination and they quickly backed away 
>from it.
>
>A few years later they brought out the PORTRAIT Plastigmat, and the 
>Portrait Unar.  While the Portrait Plastigmat was a different animal 
>entirely, the portrait unar was the same design with an expensive  
>mechanism to shift the second and negative element back and forth.
>
>Les
>On Wednesday, August 21, 2002, at 10:51 PM, Richard Knoppow wrote:
>
  The Bausch patent is interesting. I was issued a year after Paul
Rudolph's patent for the Zeiss Unar. The air spaced lenses discussed in the
Bausch patent are different from the Zeiss Unar, which has a double Gauss
back component. The double Gauss lens uses two meniscus lenses. The Bausch
patent shows only one meniscus element, a postive one at the front of both
variations.
  The cemented version looks a lot like a Dagor with a split rear element.
  The Zeiss Double Protar, which also consists of four cemented elements in
each half, was designed by Paul Rudolph to provide correction for coma in
each cell. The Dagor, and similar lenses, like the Orthostigmat, have cells
which are not corrected for coma, the lens relying on symmetry to correct
it in the combined lens. This lack of coma correction means that a single
Dagor cell will provide a reasonably sharp image only when stopped down to
very small stops, perhaps f/45. Even though the Dagor was patented and sold
as a convertible lens it is not truely convertible because of the lack of
comatic correction.
  Bausch does not mention comatic correction anywhere in the paper although
the individual cells of the cemented Plastigmat may in fact be so corrected. 
  Since Zeiss licensed B&L to manufacture its lenses in the US in 1898,
before the Bausch patent was issued, and since the Zeiss Unar patent
preceeds the B&L patent by a year, I suspect that any lens sold by B&L as a
Unar was the Zeiss version. The Zeiss Unar was not a very satisfactory lens
and was replaced by the Tessar after about 1902, although it continued to
be offered by B&L for a couple of years after that. 
  The Plastigmat was also patented after the Zeiss Convertible Protar. The
patent specifies the arrangement of powers of of the elements and their
relative indexes. These are different from the Convertible Protar so the
patent could be issued. Again, the four element-air spaced version of the
lens in the Bausch patent is quite different from the Zeiss lens so there
is no interference. 
  Its likely the four element lens is sensitive to element spacing, most
lenses are. So, one could expect a lens with variable softness, probably
from spherical aberration, from a lens with movable elements. Most lenses
of this type are Triplets with adjustable center element positioning. 
  The prior existence of the Convertible Protar plus the fact that B&L was
licensed to make it opens speculation as to why the Plastigmat was
designed. Perhaps Edward Bausch just wanted to get his name on a patent. 
  Evidently, these lenses were made for only a few years. 
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com