[LargeFormat] Happy Holidays and Zeiss Convertibles!

[Clive Warren]

At 5:02 pm -0800 22/12/02, Richard Knoppow wrote:
big snip
>
>>Many convertible lenses suffer from poor performance from
>>their single cells as they were designed to give optimal performance
>>when combined. The Series VII Protars are a little different in this respect
>>so I'm looking forward to using the lens. It's amazing that a lens over
>>100 years old will probably give even modern glass a good run for the money -
>>it's not too fast though at f12.5
>
>   I suspect there were other reasons than manufacturing
>capacity to the licensing agreements Zeiss and Goerz had.
>Import duties and cost of shipping may have been factors.
>The Zeiss Convertible Protar Series is individually
>corrected for coma. In symmetrical lenses, like the Dagor,
>symmetry is relied on for correction of coma, geometrical
>distortion, and lateral color. This correction is, of
>course, lost when single cells are used. While the
>individual cells of the protar are not corrected for lateral
>color or distortion the correction for coma makes a
>substantial difference in their performance. My experience
>is that the cells have little color fringing. To minimise
>distortion and field curvature the cells should be used
>behind the diaphragm. However, since meniscus lenses have a
>slight retrofocus effect when used this way it is sometimes
>advantageous to use them on the front of the iris to reduce
>the amount of bellows draw required. The difference in
>performance is not great and the difference in bellows draw
>is significant.
>   I've found that the image quality from individual Protar
>cells is surprizingly good. The combined lens is excellent.
>   While the Dagor was patented and sold as a convertible its
>really not because the individual cells deliver acceptable
>quality only when stopped down to practically a pinhole
>(f/45 maybe). Protar cells do well at f/32 or even f/22.
>
>---
>Richard Knoppow
>Los Angeles, CA, USA
>dickburk@ix.netcom.com

Richard,

Thank you for the clarification that it is coma that is corrected for 
in the individual cells and the further info. Minimising distortion 
by using the lens cells behind the diaphragm makes perfect sense to 
me - the lens will be used on an 8x10 camera with a 5x7 back (for the 
combined 10" focal length). The individual cells are 19 1/4" and 16 
1/2" so may well cover 8x10. The combined cells seem to have a circle 
of illumination that covers 8x10 at 1:1. The lens has a rotating 
engraved aperture scale that lines up with a side lever used to vary 
the diaphragm. Quite an elegant solution and still legible!

Ross must have used very good balsam cement as there are no signs of 
separation - although it may have been re-cemented at some point in 
its life. It certainly has that wonderful patina showing it has been 
well used. If only it could tell it's stories or perhaps show some of 
the photos that have been taken with it over the years.....

Cheers,
        Clive