[LargeFormat] An 11x14 Ektachrome shot with rear half of a Protar VII convertible

[Richard Knoppow]

----- Original Message ----- 
From: "Jim Hemenway" <Jim@hemenway.com>
To: <largeformat@f32.net>
Sent: Monday, October 13, 2003 11:10 AM
Subject: Re: [LargeFormat] An 11x14 Ektachrome shot with
rear half of a Protar VII convertible


> I can see one guy drinking a Cooors but I don't know how
he found one in
> Boston. :-)
>
> Richard, thanks for the compliment and for the info.  I
used the rear
> cell behind the shutter/aperture and wonder if I used the
front cell
> only, and in the front, is less optimal correction the
only result?
> Or, would the resulting photograph take on a different
"look" since the
> front cell would be facing forward instead of backward as
does either
> cell if used behind the aperture?
>
> TKS,
>
> Jim
>
  The "correct" position for either cell is behind the
diaphragm. The stop position influences coma, spherical
aberation, and field flatness. However, I don't know if the
stop position is optimum. I suspect not since it would have
to vary with focal length.
  Mensicus lenses have their principle points displaced
toward the convex side of the lens, so, a meniscus with its
convex side facing the film is slightly retrofocus and one
with its convex side facing the object is slightly
telephoto. So, the required bellows draw can be
significantly less for a single cell facing the "wrong" way.
Since the coverage angle of a long lens is limited the
difference in aberrations for the lens in this position may
be negligible. It may make a lens usable where its bellows
requirement is right at the limit of the camera.
  The difference is not enough to change the "look" and may
not be easily noticable.
  Where one has a triple convertible, that is two or more
cells of different focal length, the longer FL should be
used in front. I actually havn't checked this for bellows
draw but there may be some, I think here probably not of
practical interest.
  Paul Rudolph designed the Protar sections especially to be
usable alone with good image quality. They are corrected for
 coma and evidently pretty well corrected for lateral color.
In a symmetrical lens, like the Dagor, correction for coma
and lateral color is automatic because of the symmetry.
While Dagors were patented and sold as convertibles the
image quality is not very good unless stopped down to f/45
or smaller. There is probably not a great difference between
_complete_ Protars and Dagors, although it seems to me from
my lenses that the Protar may have a little less zonal
spherical (focus shift). However, this is a fault inherent
in thick meniscus lenses so neither can be free of it.
  A friend who is an actual lens designer ran the revised
Protar design in a computer lens design program and found
the program could not significantly improve it even with
changes of glass type. Pretty remarkable.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com