[rollei_list] Re: OT: Feedback on early 240mm f6.8 Goerz Dagor
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 8 Feb 2005 08:52:39 -0800
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, February 08, 2005 8:29 AM
Subject: [rollei_list] Re: OT: Feedback on early 240mm f6.8
Goerz Dagor
>I think Dagors of this vintage were a pair of symetrical
>cemented triplets. Because each cell is an anastigmat, I
>believe you can use it individually (well stopped down) for
>a different fl. Protars of this vintage were of similar
>construction.
>
> They have very good coverage and fine performance stopped
> down (but if I am remembering correctly some focus
> shift?). If it is clean, you should get nice contrast and
> image quality... sounds like great fun!
>
>
> Eric Goldstein
>
Correct, I just sent a detailed post. The Convertible
Protar (Series VIIa) consisted of four cemented elements
instead of the three of the Dagor. The difference is that
the individual Protar cells are corrected for coma. This
makes them sharper and able to work at larger f/stops than
Dagor cells. However, there is no significant difference in
combined lenses.
Dagors are symmetrical, i.e., both cells of the same
focal length. Convertible Protars can combine different
focal lengths so a triple convertible configuration is
possible. Convertible Protars were offered in sets, usually
of four cells of different focal lengths along with a
shutter and a barrel. Protar sets made by Bausch and Lomb
also included a Series V Extra Wide Angle Protar (f/18) with
an adaptor plate. This does not seem to have been offered in
the German sets.
There were alternative arrangements of elements of both
Dagor and Convertible Protar type lenses. Zeiss and Watson
offered a Dagor type with reversed order of power (-+-
instead of +-+). The Schneider Angulon is also of this type.
The Voigtlander Kollinear and Steinheil Orthostigmat
(Identical design and shared patent) were ++-. There is no
practical difference in the performance of these variations.
There were five variations of the four element type lens,
again with little or no practical difference in performance.
Zeiss Convertible Protars (and also the B&L version) are
very good lenses. The performance of the single cells is
quite surprizingly good.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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