[LargeFormat] Double Protar Lens

[Clive Warren]

At 4:02 pm -0700 1/9/03, Richard Knoppow wrote:
Huge snip
>  >
>>  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

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