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Oddity with Protar VII



 
 
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  #1  
Old October 16th 11, 06:48 PM posted to rec.photo.equipment.large-format
Richard Knoppow
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Posts: 752
Default Oddity with Protar VII


"Cheesehead" wrote in message
...
I was looking @ the rear cell on my B&L lens, and it's 23
1/4" f/l.
The front cell is 18 7/8" f/l.
The old B&L table never put a longer cell in the rear.
http://www.cameraeccentric.com/html/...schlomb_3.html
I got the camera from the original family owner, and it
was apparently
ordered this way in 1923.
Makes me wonder ... any thoughts on this anomaly?

Either cell will fit on either end, somone just reversed
them from normal. The correction is slightly better when the
longer lens is in front. When the single cells are used
alone they perform best behind the diaphragm but there is
really little difference in correction. Since the principle
points of the single cells are displaced in the direction of
the curvature the distance from the focal plane is less when
the lenses are mounted on the front of the shutter. This
allows some lenses to be used on bellows which are not long
enough to focus them when on the back of the shutter. The
difference is significantly more than just the mechanical
length although that also counts.
Both cells should have the same threads and both sides
of the shutter should be the same.
Both B&L and Zeiss sold Protars in sets with up to four
cells of different focal lengths which could be combined or
used alone. The most elaborate B&L set also came with an
extra-wide-angle Protar.


--
Richard Knoppow
Los Angeles, CA, USA



  #2  
Old October 18th 11, 01:08 AM posted to rec.photo.equipment.large-format
Jean-David Beyer
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Posts: 245
Default Oddity with Protar VII

Richard Knoppow wrote:

Either cell will fit on either end, som[e]one just reversed
them from normal. The correction is slightly better when the
longer lens is in front. When the single cells are used
alone they perform best behind the diaphragm but there is
really little difference in correction. Since the principle
points of the single cells are displaced in the direction of
the curvature the distance from the focal plane is less when
the lenses are mounted on the front of the shutter. This
allows some lenses to be used on bellows which are not long
enough to focus them when on the back of the shutter. The
difference is significantly more than just the mechanical
length although that also counts.


I have a Wisner Convertible Plasmat set with five f/13 elements.
If you use two at a time, the aperture is somewhat larger; e.g.,
if you use both 250mm elements, the maximum aperture is f/9 and
the focal length is 152mm. If you use the 400mm and 450mm elements
together, the maximum aperture is f/9.6 and the focal length is
258mm.
The cells are two 250mm, one is 350mm, one is 400 mm, and one is 450mm.

If you use just one element, he says to place it behind the diaphragm.
If you use two, he say to put the longer lens behind and the shorter
one in front. Of course it makes no difference if you use both 250mm
elements together.

When you say the correction is slightly better with the longer element
in front, do you mean in general, or just the Protar design?

--
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  #3  
Old October 22nd 11, 09:02 AM posted to rec.photo.equipment.large-format
Richard Knoppow
external usenet poster
 
Posts: 752
Default Oddity with Protar VII


"Cheesehead" wrote in message
...
On Oct 16, 1:48 pm, "Richard Knoppow"
wrote:
"Cheesehead" wrote in message

...I
was looking @ the rear cell on my B&L lens, and it's 23
1/4" f/l.
The front cell is 18 7/8" f/l.
The old B&L table never put a longer cell in the rear.
http://www.cameraeccentric.com/html/...schlomb_3.html
I got the camera from the original family owner, and it
was apparently
ordered this way in 1923.
Makes me wonder ... any thoughts on this anomaly?


Either cell will fit on either end, somone just reversed
them from normal. The correction is slightly better when
the
longer lens is in front. When the single cells are used
alone they perform best behind the diaphragm but there is
really little difference in correction. Since the
principle
points of the single cells are displaced in the direction
of
the curvature the distance from the focal plane is less
when
the lenses are mounted on the front of the shutter. This
allows some lenses to be used on bellows which are not
long
enough to focus them when on the back of the shutter. The
difference is significantly more than just the mechanical
length although that also counts.
Both cells should have the same threads and both sides
of the shutter should be the same.
Both B&L and Zeiss sold Protars in sets with up to four
cells of different focal lengths which could be combined
or
used alone. The most elaborate B&L set also came with an
extra-wide-angle Protar.

--
Richard Knoppow
Los Angeles, CA, USA


Thanks.
I've also gotten the impression that the Protar is the same
optical
formula as the old casket lens sets.
There were a number of lenses available in "casket"
sets, both Zeiss and Bausch & Lomb sold such sets but other
variations on the design were also available. Rudolf
Kingslake shows five variations of five element cemented
lenses in his book on lens history. All have similar
performance. The Turner-Reich lens, which was probably
designed by Ernst Gundlach is a five element cemented lens
very similar to the Series 7 Protar but with one of the
elements split, perhaps to get around the Zeiss patent. The
T-R lens was also available in sets. Its performance is not
as good as the Zeiss or B&L protar but lots of triple
convertible T-R lenses were built on military contract
during WW-2.
The advantage of the Protar Series VII over the Dagor
is that the individual cells are corrected for coma so they
are sharper over a considerably wider angle than a single
Dagor cell. The Dagor as a combined lens is free of coma
because of the symmetry. A combination of Protar cells is
further corrected for coma by the symmetry or near symmetry
even when cells of different focal length are combined.
While none of the single cells is as good a lens as a
complete lens they were good enough when work was done on
large format which was not enlarged or at not enlarged much.
They were an economy where a commercial photographer wanted
to have the resources to do a variety of work. B&L included
an extra-wide-angle Protar in their largest combination sets
with either a Volute shutter or an adaptor plat so that it
could be used on the Compound shutter usually supplied with
the set, as a front shutter.
There are some old Zeiss and B&L catalogues at
http://www.cameraeccentric.com which show the variety of
protar lenses sold by both Zeiss and B&L and, I think, also
a Gundlach catalogue or two. Ernst Gundlach discovered the
method of splitting elements of existing designes as a way
of bypassing patents pretty early. I don't think much of
Gundlach lenses but the company also made Korona cameras
which were pretty good.



--
Richard Knoppow
Los Angeles
WB6KBL



 




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