[LargeFormat] Happy Holidays and Zeiss Convertibles!

[Richard Knoppow]

----- Original Message -----
From: "Clive Warren" <Clive.Warren@megacycle.co.uk>
To: <largeformat@f32.net>
Sent: Sunday, December 22, 2002 1:58 PM
Subject: [LargeFormat] Happy Holidays and Zeiss
Convertibles!


> Dear All,
>
> The list has been unusually quiet this weekend - guess you
are all out
> there doing your seasonal shopping.
>
> I have been hit by a flu bug that has really taken me out
over the last
> couple of weeks. If I owe anyone EMail then please  be
patient as there is
> rather a lot to catch up on.
>
> As usual, I will be travelling around the country over the
next few days
> and hope to be taking photos. There are a couple of shots
that would be
> good to revisit..... In addition there are a few lenses
that need to be
> tried out, including a Ross Convertible Anastigmat that
surfaced here
> recently. This lens was manufactured in around 1896 by
Ross in the UK under
> licence from Zeiss. At the time, Zeiss did not have the
manufacturing
> capabilities to produce any quantities of lenses so simply
licenced out
> their patents to lens producers in various countries. In
the US it was
> Bausch and Lomb, in France it was Krauss. The lens was
later renamed the
> "Double Protar" or Series VIIa Protar and it is as you may
have guessed, a
> convertible. The Anastigmats are composed of two lens
cells each containing
> a completely corrected  4 element symmetrical anastigmat
lens.
>
> 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
>
> Anyway, hope you all have an enjoyable holiday and a happy
New Year!
>
> Cheers,
>             Clive
>
>
  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