[rollei_list] Re: Lens recommendation
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 3 Aug 2010 07:58:57 -0700
----- Original Message -----
From: "CarlosMFreaza" <cmfreaza@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, August 03, 2010 5:38 AM
Subject: [rollei_list] Re: Lens recommendation
Thanks Richard for the explanation and the difference about
longitudinal chromatic aberration and lateral color
aberration
specially, and then the "Apochromatic" DIN standard is/was
for the
lateral color aberration while the traditional Abbe
definition is for
the longitudinal color aberration.
And then if the lateral color aberration is proportional to
the focal
length and a lens reduces this proportion to a minute
percentage of
its focal length, this lens is "apochromatic" from this
point of view,
it does not mean it is also apochromatic for the
longitudinal color
aberration, it could be or not to be and vice-versa. It
becomes also
clear that the lateral color aberration is more significant
for long
tele-lenses as the Schneider brochure states.
Carlos
Well, since all aberrations scale with focal length the
statement about lateral chromatic is a bit misleading. Both
kinds of chromatic aberration are a constant percentage of
the image as focal length changes but what is important is
when only a small part of the image is used as in a long
focus or telephoto lens. Then the aberrations become
magnified along with the image. A long lens for small format
may be easier to correct since its coverage angle is small
but the corrections have to be good since the image is
magnified.
Its been a very long time since color blind and
orthochromatic films were in general use so chromatic
correction of both kinds has had to be pretty good for
nearly a century. In the days when films were not sensitive
to red light achromatic correction could be concentrated in
the yellow to blue range but panchromatic films and color
films require minimum focus deviation over at least the
visible range of colors. Getting good lateral chromatic
correction is easy when the lens can be symmetrical or
nearly so but becomes much more difficult for
non-symmetrical lenses especially for telephoto and reverse
telephoto lenses. They _can_ be corrected very well but the
designer does not have the convenience of automatic
correction or partial correction from symmetry. I should
note that perfect cancellation of the lateral aberrations,
coma, geometrical distortion, and lateral color, is obtained
only when the entire optical system is symmecrical, that is,
at unity magnification. This, or near it, is the usual
condition for a process lens and if the lens itself is
symmetrical as in an Apochroatic Artar, the image will be
free of these faults. However, longitudinal chromatic must
still be corrected in the usual way by balancing the partial
dispersions and powers of the glasses used. Of course the
other aberrations must also be corrected. Chromatic errors
also show up in the correction of spherical aberration so
getting a lens to have good definition for a range of colors
is not a trivial problem.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
Rollei List
- Post to rollei_list@xxxxxxxxxxxxx
- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe'
in the subject field OR by logging into www.freelists.org
- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with
'unsubscribe' in the subject field OR by logging into www.freelists.org
- Online, searchable archives are available at
//www.freelists.org/archives/rollei_list
Other related posts:
