[LargeFormat] Convertible lenses

[Kent Gibbs]

By the way, thanks to Richard and Ole for the info on convertible
lenses.  Must have been one of those holiday moments.

Kent


--- Richard Knoppow <dickburk@ix.netcom.com> wrote:

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 have a newer version of the Symmar but evidently the 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