[LargeFormat] Convertible lenses

[Richard Knoppow]

----- 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have a newer version of the Symmar but evidently the 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