[rollei_list] Re: Lens recommendation
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 1 Aug 2010 15:34:12 -0700
----- Original Message -----
From: "CarlosMFreaza" <cmfreaza@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, August 01, 2010 1:55 PM
Subject: [rollei_list] Re: Lens recommendation
2010/8/1 Elias_Roustom <elroustom@xxxxxxxxx>:
What is the significance of the APO designation. I see
lots of Rodagon
lenses without the APO.
The German standards "D.I.N" definition for an
Apochromatic lens is
that the lateral chromatic aberrations of the secondary
spectrum are
reduced to within a minute percentage of the focal length
of the lens,
it's the main reason several German lenses have the APO
designation,
some Japanese lenses also use this designation. The DIN
definition
looks different regarding the traditional Apochromatic
definition,
basically: "Apochromat: A lens in which light of three
colors is
brought into common focus", PHOTOGRAPHIC OPTICS by Arthur
Cox 15th
edition 1974.
And then if you take the traditional definition, most APO
lenses are
not Apochromatic, the German lenses like the Apo Rodagon,
Apo Sironar
and most others "Apo" are Apochromatic according the DIN
definition,
the DIN definition looks more "tolerant" regarding the
traditional
definition. Some tests about these DIN Apochromatic lenses
show they
are not Apochromatic for the term traditional definition.
Elias, I live within a sub-tropical area and fungus can't
develope a
web so quickly IMO, perhaps your lens already had little
white points
that became web. I read Olympus and Zeiss and Schneider
recommendations to prevent them: Silica Gel, to change the
air around
your lens often and to allow them to receive the bright
sunlight
regularly
Carlos
I had a long argument with Bob Salomon, the agent for
Rodenstock, about this. It turns out that the APO lenses are
no such thing. They are just well corrected achromats but a
badly written DIN standard allowed them to be called
Apochromatic. As you state the definition of apochromatic is
very well established in optics but has been applied mostly
to telescope and microscope objectives. It means a lens
which is corrected for longitudinal chromatic aberration for
three colors and for spherical aberration at two colors.
Usually one wants a lens with minimal lateral chromatic but
that is not corrected simutaneously with longitudinal
chromatic and is not part of the definition. I can't
remember if it is Rodenstock or Schneider that publishes
longitudinal chromatic aberration curves but they show
clearly that the APO lenses are achromats. An achromatic
lens is corrected for longitudinal chromatic aberration at
two colors and for spherical at one color. the cuve of the
focus vs: wavelength is hump shaped with two zero crossings.
An apochromatic lens has an S shaped curve with three zero
crossings. In theory its possible to correct a lens for any
number of colors, those corrected for more than three are
called superachromats, they are quite rare and my lens
designer friend Brian Caldwell says it doubts if any genuine
apochromatic lenses exist. I don't understand his reasons
for this.
In the past most apochromatic photographic lenses were
made for three-color process work so that the three color
plates would all be in focus and of the same size. The
latter requires that the lenses be corrected for lateral
chromatic. One way to obtain this correction is though
symmetry. A symmetrical lens in a completely symmetrical
optical system (equal object and image distances or, in
other words unity magnification) has automatic correction
for the three lateral aberrations, lateral color, coma, and
geometrical distortion. For simple dialyte lenses of the
Apochromatic Artar type the correction stays pretty good
even at infinity focus.
Non-symmetrical lenses, such as the retrofocus lenses
common on SLRs can also be corrected for lateral color but
they are much harder to design.
My understanding is that the DIN standard was
re-written to preclude the naming of achromats apochromatic
but I am not sure of this and the practice continues.
Even though some of these lenses my be exceptionally
well corrected with minimum color focus deviation, they are
still not apochromatic lenses and calling them that is IMO
fraud.
BTW, Kingslake points out that a well designed achromat
may have smaller deviation between its corrected points than
a poor apochromat. Being an apochromatic lens is, by itself,
no gurantee of quality.
--
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:
