[rollei_list] Re: : Schneider enlarger lenses
- From: ERoustom <eroustom@xxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Fri, 8 Jun 2007 10:19:48 -0400
Fascinating! Thank you Richard.
What should I be reading to learn more about this, or is it strict
physics, and I'd have to smarten up a bit first?
Elias
On Jun 7, 2007, at 10:16 PM, Richard Knoppow wrote:
----- Original Message ----- From: "ERoustom" <eroustom@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, June 07, 2007 6:26 PM
Subject: [rollei_list] Re: : Schneider enlarger lenses
I got my Componon "upgrade" in the mail today, and thought I'd
share a visual on the difference between it and the Componar:
http://farm2.static.flickr.com/1076/535268700_a8b6ca5c91_o.jpg
More glass area - is the obvious difference. Is that what makes
the Componon a better lens?
Elias
Its not so simple as that. Part of the difference in in the
number of degrees of freedom the designer has to correct the
optical errors, called aberrations in the lens. The simplest lens
that allows all the basic aberrations to be corrected is the Taylor
Triplet, which has three elements or lenses in it and six surfaces.
Since all the variables are needed to correct the fundamental
aberrations and establish the focal length its hard to correct a
Triplet for higher order aberrations. They can be very good for a
relatively narrow field of coverage or at slower speeds but have
problems when the field is widened or the speed increased. A Tessar
type lens has four glass elements, two of which are cemented
together. This gives the designer a additional surface to help with
the corrections. Since the surface can be "bent" there are two
additional degrees of freedom over the Triplet.
The Componon is a type of lens known generically as a Plasmat.
The simplest Plasmat has six lenses with a cemented pair in each
half. The Plasmat can be very highly corrected for the higher order
aberrations plus it has some degree of symmetry which partially
corrects for three important aberrations, namely lateral color,
geometric distortion, and coma. Coma is an effect where off-axis
point become tear-drop shaped blur spots. The Plasmat makes a good
semi-wide angle lens, has flat field, and can be especially well
corrected for an aberration called astigmatism. This is the not the
same problem as astigmatism in the human eye but rather a proplem
where radial lines and coaxial lines do not come to a common focus
at all parts of the image. Because this can be well corrected in
this type of lens the lens can also be made to have an unusually
flat field, an important property in an enlarging or copying lens.
There are many aspects to the design of a lens, it is far from a
trivial excercize. Modern lens design is done with the help of
computer programs. While these can solve some very complex problems
they must still operate under the guidance of a skillful lens
designer in order to come up with practical designs.
A big part of the choice of a particular design is cost. The
more complex the lens, in general, the more it will cost to make.
Some designs are very critical of mechanical tollerances which,
again, makes them expensive. Some designs rely on special optical
glass which may be very expensive. Designers also try to avoid
cemented surfaces because the mating surfaces must be polished to a
very close tolerance to fit exactly and the cementing is hand work
requiring several extra steps.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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