[rollei_list] Re: Lens recommendation
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 4 Aug 2010 08:38:59 -0700
----- Original Message -----
From: "CarlosMFreaza" <cmfreaza@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, August 04, 2010 3:28 AM
Subject: [rollei_list] Re: Lens recommendation
.. It becomes also
clear that the lateral color aberration is more
significant for long
tele-lenses as the Schneider brochure states.
Carlos
2010/8/3 Richard Knoppow <dickburk@xxxxxxxxxxxxx>:
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. ...
Richard, I don't think all optical aberrations scale with
the lens
focal length, in general they scale with the image size
because they
become more visible, but it has nothing to do with the lens
focal
length, some optical aberrations like the barrel distortion
is typical
for short focal length lenses and it tends to diminish
according you
increase the focal length.
The specific lateral or transversal color aberration has to
do with
the image magnification directly, this aberration causes
that the
image size is different for each color, thus the image
magnification
depends on the wavelength, the size of the image varies from
one color
to the next. In a lens corrected for longitudinal chromatic
aberration
the principal planes do not need to coincide for all colors.
Since the
focal length is determined by the distance from the rear
principal
plane to the image plane, the focal length may depend on the
wavelength even when all images are in the same plane.
Lateral color is specially evident in telephoto and
retrofocus lenses,
lateral color is the main cause of the separation between
the sagittal
and tangential curves in their modulation transfer
functions. The
typical manifestation of chromatic aberrations is color
fringing along
boundaries that separate dark and bright parts of the image:
http://toothwalker.org/optics/chromatic.html
Carlos
---
Well, in absolute dimensions it still scales but you are
right that its larger because the image is larger.
This is a fascinating site. I spent about forty-five
minutes looking at it. For some reason he leaves out coma,
which is an important aberration. Also, he does not
illustrate one of the properties of astigmatism, that is
that a point in the object plane is imaged as a line in the
image field. There are two points of focus for any point,
one images the point as a line segment along a radus and the
other as a line on a tangent. Between the two is a minimum
blur spot. As he points out there are two places where the
tangential and radial (he uses the alternative term sagital)
fields come together, one is at the center of the image and
the other is at some point at a given image angle where the
designer has decided it should go. This is called the
sigmatic point. One of the differences among lens designs is
the rate at which the two fields deviate beyond the
stigmatic point. For some lenses the deviation is rapid and
the image quality deteriorates very rapidly, the dialyte is
an example, for other lenses the deviation is fairly slow so
that by stopping down the field can be increased somewhat. A
Dagor is an example.
It was thought at one time that it was impossible to
achieve stigmatic correction and chromatic correction
simutaneously. That awaited the invention of the barium
glasses by Schott and Abbe. However, it was later found that
an color corrected lens which was also an anastigmat _could_
be made with all "old" type glass. My memory is that the
designer was Martin but I am drawing a blank on the name of
the lens. It is a four element air spaced type. Previous to
the invention of anastigmats the astigmatism was dealt with
by introducing field curvature to avarage the stigmatic
deviation on either side of the image plane. The increased
depth of focus from stopping down then allowed fairly sharp
images. Many Rapid-Rectilinear lenses are designed this way.
BTW, he illustrates the chromatic deviation graphs I
spoke of very well showing four conditions. BTW,
Superachromats can be corrected, at least in principle, for
more than four colors. In such a case the curve would have
as many zero crossings as the number of colors the lens was
corrected for. He also points out, as I did, that the
important thing in image quality is not the number of colors
corrected for but the maximum deviation of focus or
secondary spectrum. Modern achromats can have very small
amounts of secondary spectrum.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
---
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