[LargeFormat] Convertible lenses
Richard Knoppow
largeformat@f32.net
Thu Dec 16 14:44:45 2004
----- Original Message -----
From: "Kent Gibbs" <kent_gibbs@yahoo.com>
To: <largeformat@f32.net>
Sent: Thursday, December 16, 2004 10:51 AM
Subject: [LargeFormat] Convertible lenses
>I am considering purchasing a convertible lens to try to
>keep my kit
> manageable and would like everyone's thoughts on the
> pluses and minuses
> of these types of lenses.
>
> Thanks
>
> Kent
>
Convertible lenses are of several designs. Probably the
most common are Dagor and Zeiss Convertible Protar types.
Symmetry in a lens automatically corrects three aberrations:
coma, lateral color, and geometrical distortion.
Cancellation is complete only when the entire system is
symmetrical, i.e., image and object the same size, however,
the cancellation is substantial at any distance. When one
cell of a convertible is used these corrections are lost
unless the cells are individually corrected for them. Dagors
are not corrected individually so there is a substantial
amount of coma unless the lens is stopped down a lot. The
corners are not very sharp until around f/36, maybe even
smaller. The Convertible Protar _is_ individually corrected
for coma so its performance is much better as a single cell.
The performance as a combined lens is not much affected by
this. However, it is possible to use cells of different
focal lengths together in a Protar for a variety of focal
lengths. The coverage of the individual cells of all
convertibles is about the same as the combined lens, that
is, the angular coverage of the cells is narrow compared to
the combined lens.
Some more modern convertibles are of the Plasmat type.
The cells of my Schneider Symmar appears to be corrected for
coma and its performance is quite good. However, I think my
ancient Zeiss Convertible Protar is actually better. I don't
have a newer version of the Symmar but evidently the
performance of the non-convertible version is superior to
the convertible version. Of course, the single cells can
still be used alone and may work about as well.
Convertible sets were meant to be an economy. The modern
sets, which have afocal corrector plates, are no economy.
A great many designs of convertibles were made over the
years, they were very popular in the early part of the last
century. AFAIK the Zeiss Convertible protar is as good any
any and maybe better than most. Wollensak made a convertible
based on the Protar which is very good. A great many of
these were made for the govenment to meet a specification
for the Dagor/Protar in a focal length suitable for 8x10.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com