[LargeFormat] Convertible lenses
Kent Gibbs
largeformat@f32.net
Wed Jan 5 09:24:07 EST 2005
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 have a newer version of the Symmar but evidently the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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