[LargeFormat] Double Protar Lens

[Richard Knoppow]

----- Original Message ----- 
From: "Clive Warren" <cocam@blueyonder.co.uk>
To: <largeformat@f32.net>
Sent: Monday, September 01, 2003 3:17 PM
Subject: Re: [LargeFormat] Double Protar Lens


> At 5:08 pm -0400 1/9/03, LNphoto wrote:
> >On Sunday, August 31, 2003, at 01:19  PM, Clive Warren
wrote:
> >
> >>Have just picked up a Bausch and Lomb Series VII Protar
for a 5x7
> >>camera that appears to be a double convertible rather
than a
> >>triple. Without looking at the books, I thought they
were all
> >>triple convertibles. Does anyone have one or used one?
> >>
> >>Cheers,
> >>            Clive
> >>
> >>
> >>
> >If you bought to cells of different focal lengths you
gained a third
> >focal length at the cost of speed as most combined were
either f 7.0
> >or 7.7.   If you combined two of the same lenght your
combined focal
> >length was f6.3....modestly fast for those days.
>
> Thanks for all the replies.
>
> It seems that my confusion lay in the fact that the lens
is in a
> shutter with two aperture scales. I do not have the beast
yet, but it
> seems to be a single 11" approx. Series VII - ie half a
double
> Protar, probably placed in the shutter some time after its
1895
> manufacture. More than likely it is f12.5
>
> I have a Ross Double Protar that is a 10" f7 with 16 1/2"
and 19 3/4"
> cells at f12.5. It's engraved with a flowing script:
"Zeiss
> Convertible Anastigmat". Cleaned it up a couple of months
ago and
> when the new multi-lens board with Packard is set up will
shoot some
> film with it. It does seem as though it'll fit front
mounted on a
> Betax #5 shutter so I may get around to making a sleeve up
for
> it..... In any case, the new Bausch and Lomb lens may be a
direct fit
> in the Ross barrel so there could be a few more focal
lengths
> available.
>
> Many of these older lenses have the patina of use - from
the wear of
> the brass lens barrel, the Ross looks as though it was
lovingly used
> for many decades, possibly from an old pro. studio
locally. If we
> could only see some of the photos made with it.....
>
> Cheers,
>         Clive
>
  Zeiss licensed several companies to make its lenses at a
time when import duties made this economical. In the U.S.
the licensee was Bausch & Lomb, in France Krauss, in England
Ross.
  Bausch & Lomb was also licensed to make Deckel shutters.
  B&L lenses were not exact copies of Zeiss lenses; they had
different focal lengths and different mechanical specs
including cell threads. The Deckel shutters (Compound and
Compur) also have different dimensions and threads than
German made shutters. They are not interchangible.
  I don't know if this is true of Krauss and Ross made
lenses.
  A single cell should be used behind the stop, as I
mentioned the stop position affects several of the
aberrations.
  In fact, the speed varies with the stop position. When
used behind the stop the entrance pupil is the diaphragm and
its mechanical size is the stop size. When used in front of
the diaphragm there is a little pupilary magnification so
the entrance pupil lies somewhere in front of the stop and
the speed is increased a little. This is because the
entrance pupil is larger than the diaphragm. The difference
is small, perhaps half a stop. When used in their "normal"
position i.e., behind the stop, the speed of the individual
cells is f/12.5.
  Because the correction is not so good as a combined lens
the optimum stop for single cells is around f/32-f/36.
Coverage of single cells is about half that of a combined
lens of equal focal length, i.e. about 45 degrees.
  IMHO, from direct comparison, I think the Convertible
Protar single cells are superior to the Schneider
Convertible Symmar single cells.
  Hopefully your lens has two cells and not just one, the
double scale suggests it must have had originally.
  Zeiss Protar sets or triple convertibles came in shutters
with stop calibration in millimeters with a chart showing
the f/stop settings in mm for each lens and combination.
  The Protar was re-designed at least once. The redesign has
slightly better performance than the original.
  A friend, who is an actual lens designer, set up the
second Protar design in a computer lens design program and
discovered it is already about optimum. The program could
not improve its performance significantly. He also found
that the use of high index glasses would not significantly
improve the lens. This is true for a number of old Zeiss
designs. These folks knew what they were doing!
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com