[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>; "Dick Koolish"
<koolish@TheWorld.com>
Sent: Monday, October 13, 2003 4:29 PM
Subject: Re: [LargeFormat] An 11x14 Ektachrome shot with
rear half of a Protar VII convertible


> Thanks for the additional information Richard.  I haven't
noticed any
> focus shift at either the 13 1/4 or the 23 1/4 focal
lengths.  That is,
> if I understand the concept of focus shift correctly, I
can focus wide
> open, close the lens down to as much as f45, and with a
loupe see that
> previously focused elements have not gone out of focus.
>
> Jim - http://www.hemenway.com
>
>
  That is correct. Spherical aberration does not cancel in
symmetrical lenses, rather, it adds, so the spherical of the
individual cells is no worse and sometimes better than the
combined lens. Focus shift comes from spherical aberration.
As you know different radiuses of the lens focus light at
different distances. This is inherent in a spherical
surface. One purpose of the multiple elements in a lens is
to correct for this so that the image appears to be focused
by a non-spherical surface. The smaller the diameter of the
lens the less spherical there is. So, spherical is reduced
as the lens is stopped down. Since most people judge focus
by the image contrast the spherical aberration makes it
appear that the point of best focus moves away from the lens
as it is stopped down. If you focus for minimum spot size
this may not be so.
  Zonal spherical aberration is a higher order aberration.
It causes the anulus at about 0.707 of the lens diameter to
have more spherical than near the center (it is absent at
the center) or at the margin. The lens designer can balance
the spherical of different orders to change this curve. A
lens can be adjusted for minimum spot size when wide open,
usually the choice for lenses with a large amount of
spherical, or it can be balanced for minimum focus shift, a
good adjustment where the lens is to be focused by a
rangefinder. Most lenses are balanced so that spherical is
at a minimum in the paraxial region (very near but not at
the center) and at the margin. Usually, but not always, this
results in maximum spherical at the 0.707 point. Notable
exceptions are lenses with Merte surfaces, such as the Leitz
Hektor, will sometimes have a maximum somewhere near the 0.5
point. The numbers refer to the amount of deviation of focus
for light entering the lens at different radiuses. This
effect probably has influence on how the lens looks when
somewhat out of focus.
  In general, symmetrical doublets made from thick meniscus
elements, such as the Dagor, Series VII Protar, and many
others, have a large amount of zonal spherical. The
aberation does not seem to vary much with different
arrangements of the cemented elements.
  The Plasmat, which is derived from the Dagor by splitting
and air spacing the inside elements, allows much better
correction of zonal since it gives the designer the
possibility of varying both an additional surface and the
spacing. In effect, the space becomes an "air lens". There
are other lens forms which have less spherical than Plasmats
but modern ones (Schneider Symmar, Rodenstock Sironar, etc.)
have very good correction for it and the form can very
highly corrected for astigmatism. Most modern Plasmats are
slightly asymmetrical to improve their correction for
distant objects.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com