[LargeFormat] 480 Apo Nikkor Cell Spacing Question

[Richard Knoppow]

----- Original Message ----- 
From: "Jim Hemenway" <Jim@hemenway.com>
To: <largeformat@f32.net>
Sent: Sunday, January 25, 2004 4:38 PM
Subject: Re: [LargeFormat] 480 Apo Nikkor Cell Spacing
Question


> I have two process lenses, while they give good coverage
for 11x14 at
> infinity, there is so much of the outer areas which are
just a little
> out of focus, that they're really only good for 8x10.
>
> A retired optics guy from Polaroid, told me that the
process lenses have
> just one element less than their non-process lens twins.
This is
> because they were not meant for infinity use but instead
for 1:1
> copying... that it was cheaper to not include the missing
element since
> it wasn't needed.
>
> I don't know how true this is, but I can tell you that
I've given up on
> them for 11x14 use EXCEPT for macro work, when they are
sharp all over
> the 11x14 piece of film.
>
> Jim - http://www.hemenway.com

   I think the culprit is coma. Coma is automatically
corrected in a symmetrical optical system. By system I mean
that the lens must be symmetrical _and_ the distance to
image and object must be the same, i.e., unity
magnification. At other distances some coma appears, the
amount depending on the type of lens. Four element process
lenses of the type sometimes called a Dialyte, such as the
well known Apochromatic Artar, have rather stable
corrections with distance. These lenses have goo performance
at infinity focus but do have some coma there. Coma varies
with the square of the stop so stopping down gets rid of it
pretty quickly. At f/22, an f/9 Artar will be free of coma
at infinity. For lenses which are to be used mostly at
infinity the corrections are often optimized by shifting
some power from one cell to the other. This improves coma
correction (and some other aberrations) at infinity.
  Since this can be done by adjusting element powers or even
spacings I don't know what the extra element might have
done.
  Process lenses have rather narrow coverage. Dialytes
around 45degrees at infinity and Tessars maybe 48. The
Artar, although its rated at only 45 degrees at infinity
focus _will_ cover an image circle of a diameter equal to
the focal length when stopped down, but image quality toward
the margins is somewhat degraded. Wide angle process lenses
are usually of the Plasmat type. Again, they are more
limited in coverage than standard lenses of this type,
perhaps 60degrees rather than the 80 degrees typical of
general purpose Plasmats. I don't know which type Nikon
process lenses are. Their general purpose LF lenses are slow
Tessars.

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