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Schneider Symmar's



 
 
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  #31  
Old April 15th 04, 09:54 PM
brian
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Default Schneider Symmar's

(jjs) wrote in message ...


Are there ANY TRUE APOs in the world, and if there were would they be
suitable for general LF work (which means not flat-art, and not just work
significantly closer then infinity) ???

Would a 'real' APO really be significantly better than an adequate LF
lens? Are we not splitting hairs?

IMHO it's manufacturing quality control that is at issue with lenses. The
variations between individual lenses is more daunting to me than finding
'the best' in terms of categorical optical-bench ideals.

It seems to me that most relatively new LF users would get finer results
if they learned to focus and expose properly before they criticized the
lens they use.


I've seen some Artar type designs that actually are apochromatic -
that is with three crossings in the axial color curve. I'm not sure
if actual Artar or Ronar process lenses have this level of correction,
however. As far as I know, none of the currently manufactured large
format lenses are apochromatic. The "apo" sironar/symmars certainly
are not. For most purposes it does not matter.

It is possible to design a Plasmat having true apochromatic
correction, but unless you do extensive work in the infrared or
ultraviolet you would probably not notice much improvement. A true
large format apochromat could be put to very good use if you
photograph artwork at discrete wavelengths over a large waveband, for
example.

Apochromatic correction is pretty much independent of field curvature
correction. Also, symmetrical or quasi-symmetrical apochromats will
generally retain true apochromatic correction regardless of
magnification. The aberration that typically restricts the
magnification range of a large format lens is coma.

Another area where true apochromatic correction is highly desirable is
with long focal length lenses which must have extremely good
correction. Examples range from broadcast zoom lenses to large
refracting telescopes. People might be surprised that zooms can be
apochromatic, but its a fact that the zoom lens with the worlds
largest zoom range (300:1) is also a true apochromat with three color
crossings in the longer focal lengths. And in case Bob is reading,
this is *not* a microscope objective!!

Brian
www.caldwellphotographic.com
  #32  
Old April 15th 04, 09:58 PM
jjs
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Default Schneider Symmar's


"brian" wrote in message
m...

Symmetry alone can achieve perfect lateral color correction at 1:1 in
a lens which is not even achromatic. The DIN spec is nonsense, and
would not be acceptable to anyone I know in the lens design/optics
community.

Brian
www.caldwellphotographic.com


Now I'm really confused. When you say 1:1, do you mean original image size
to rendered film image size? If that's what you mean, how would that apply
to general LF photography?


  #33  
Old April 16th 04, 01:01 AM
AArDvarK
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Default Schneider Symmar's

Bob, the DIN is German created, they can, I am sure,
create any standard they want as national* for the purpose
of MARKETING, maybe we should all take a look at
the exact ISO of what apochromatic is? The German
point in doing so is a bit obvious, which is that it is more
than likely that it is more expensive to make true APO
lenses (to the exact science of it) than to make achromats
and simply call them APO's. They are lying (only my
suspicion at my level of knowledge).

Alex


  #34  
Old April 16th 04, 01:05 AM
AArDvarK
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Default Schneider Symmar's

That kind salesmanship as a forthright mentality is like
the tinsel on an artificial christmas tree, rediculous.
"DIN" standards, please ...

Alex


  #35  
Old April 16th 04, 01:28 AM
Bob Salomon
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Default Schneider Symmar's

In article 1nFfc.8103$432.4878@fed1read01,
"AArDvarK" wrote:

Bob, the DIN is German created, they can, I am sure,
create any standard they want as national* for the purpose
of MARKETING, maybe we should all take a look at
the exact ISO of what apochromatic is? The German
point in doing so is a bit obvious, which is that it is more
than likely that it is more expensive to make true APO
lenses (to the exact science of it) than to make achromats
and simply call them APO's. They are lying (only my
suspicion at my level of knowledge).

Alex


Fine. Try this then:

"


Defining Apochromatism



by Thomas Back




With the proliferation of apochromatic refractors that are available
to the amateur astronomer, it is time to define the parameters of a
true apochromatic lens.

The modern definition of "apochromat" is the following:

An objective in which the wave aberrations do not exceed 1/4 wave
optical path difference (OPD) in the spectral range from C (6563A -
red) to F (4861A - blue), while the g wavelength (4358A - violet) is
1/2 wave OPD or better, has three widely spaced zero color crossings
and is corrected for coma.

Here is a more detailed analysis for those that are interested. The
term "Apochromat" is loosely used by many manufacturers and amateurs
astronomers. Let's look at the history of the definition, and maybe a
more modern one. Ernst Abbe, in 1875, met and worked for Carl Zeiss,
a small microscope, magnifier and optical accessory company. They
realized that they needed to find improved glass types, if they were
going to make progress with the optical microscope. In 1879, Abbe met
Otto Schott. Together they introduce the first abnormal dispersion
glasses under the name of Schott and Sons.

Abbe discovered that by using optically clear, polished natural
fluorite, in a microscope objective, that apochromatism could be
achieved. These first true apochromatic microscope objectives were so
superior to the competition, that Zeiss gained nearly the entire high
end market.

So secret was the use of fluorite, that Abbe marked an "X" on the
data sheet for the fluorite element, so as to keep it secret from the
other optical companies. Abbe's definition of apochromatism was the
following.

Apochromat: an objective corrected parfocally for three widely spaced
wavelengths and corrected for spherical aberration and coma for two
widely separated wavelengths.

This definition is not as simple as it sounds. I have designed
thousands of lenses: simple achromats, complex achromats, semi-apos,
apochromats, super-achromats, hyper-achromats, and Baker
super-apochromats. Abbe's definition, to put it in clearer terms (I
hope) is that a true apochromat is an objective that has three color
crossings that are spaced far apart in the visual spectrum (4000A,
deep violet to 7000A, deep red). However, just because a lens has
three color crossings, doesn't mean that it is well corrected. Let's
say that a 4" lens has three color crossings at the F, e and C
wavelengths (4861A, 5461A and 6563A). Fine, this objective is now
considered an apochromat because it has three color crossings in the
blue, green and red. But what about the level of spherical aberration
at each of these wavelengths? If the lens is 2 waves overcorrected at
4861A, and 1.5 waves undercorrected at 6563A, is it still an
apochromat? No. It is no better than an achromat, as the OPD
wavefront error is worse than a 4" f/15 achromat. Abbe, in his
definition of apochromat, states that spherical aberration must be
corrected for two widely spaced wavelengths.

Now I can tell you what happens when you correct spherical for two
widely spaced wavelengths; you correct for all the wavelengths
between them too. This is called correcting for spherochromatism (the
variation of spherical aberration with a change in wavelength). Only
with extremely long focal lengths, aspherics, large air spaces, or a
combination of the three, can you correct for this aberration.

It is the designer that must come up with a good compromise of color
correction, lack of spherical aberration (3rd order and zonal) and
control spherochromatism, so as not to degrade the image contrast. Al
Nagler used a wide air-spaced Petzval design with Fluorite and
Lanthanum glass in his TV-140 to control the above aberrations.
Roland Christen uses the highest quality super ED glass (FPL-53) and
specially matched crowns to control the various aberrations (he also
slightly aspherizes the outer surfaces). TMB Optical uses Russian
OK-4 super ED glass (similar to FPL-53) with an outer crown and a
special dense crown glass, using air spacing with different internal
radii to control the above aberrations.

Also, the Abbe condition of coma correction is overstated, that is,
if a lens is well corrected for coma at one wavelength, in almost all
cases it will be corrected for coma at all the visual wavelengths.

Now you might ask, what is a modern definition of apochromatism?
Well, you might begin with three color crossings, but you would be
wrong. One of the first things an optical designer discovers is that
with catalog glass data, it is easy to design lenses with three or
even four color crossings (super-achromat). But when you get 6 place
or greater melt data, these designs often breakdown to only two or
three color crossings (that is not to say that a 4 color crossing
objective cannot be made), albeit with the chromatic focal shift
being very small. What is really important is how small the chromatic
focal shift is (not the zero crossings) over a wide spectral range,
and how low the spherical aberration is over that same range. So we
are left with quite an ambiguous definition.

After designing, testing and selling many different apochromatic
lenses I can state this: There is no "definite" line where a lens
becomes apochromatic (in the world of commercial APO lenses). But any
lens, be it a doublet, triplet, air-spaced or Petzval, that has a
peak visual null (~5550A - the green-yellow) with a Strehl ratio of
..95 or better, coma corrected and is diffraction limited from C (red)
to F (blue) with 1/4 wave OPD spherical or better, spot sizes under
the diffraction limit (about 10 microns in an f/8 system), has good
control of the violet g wavelength with no more than 1/2 wave OPD P-V
spherical and a spot size no larger than about 3x the diffraction
limit, will satisfy the modern definition of "Apochromatism."

Lenses of this quality do not satisfy the Abbe definition, but for
all intents and purposes, will be color free and will give extremely
sharp and contrasty images.

Thomas Back
TMB Optical"

--
To reply no_ HPMarketing Corp.
  #36  
Old April 16th 04, 03:36 AM
brian
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Default Schneider Symmar's

"jjs" wrote in message ...
"brian" wrote in message
m...

Symmetry alone can achieve perfect lateral color correction at 1:1 in
a lens which is not even achromatic. The DIN spec is nonsense, and
would not be acceptable to anyone I know in the lens design/optics
community.

Brian
www.caldwellphotographic.com


Now I'm really confused. When you say 1:1, do you mean original image size
to rendered film image size? If that's what you mean, how would that apply
to general LF photography?


1:1 means unit magnification, i.e. the image is the same size as the
object. The point I was making was that any symmetrical lens will
automatically be corrected for lateral color at 1:1 magnification.
Bob's "apo" definition talks about reducing *lateral* color at 1:1.
By this definition even a lens with primary chromatic aberration (i.e.
not even achromatic) could be called "apo".

Therefore, its absolutely ludicrous to define "apo" by talking about
lateral color at 1:1.

To be honest, because I work as a professional lens designer I
probably get alot more upset than most people when I see marketeers
and others suck the meaning out of a well established optical term.

Brian
www.caldwellphotographic.com
  #38  
Old April 17th 04, 12:16 AM
AArDvarK
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Default Schneider Symmar's


Now THAT is a good lesson, and a serious understanding, Bob!
His writing suggests it is genuinly impossible to create a true
apochromatic lens anyway, because of natural production
circumstances (for shorter lenses). I still do not have the
understanding tech-wise but some things are obvious. I still
would like to know what you know of the genuine APO quality
differences between Rodenstock and Schneider, if you know
of any, even if you are the distributer of Rodenstock. Are you
also the importer?

Alex


  #39  
Old April 17th 04, 12:46 AM
Bob Salomon
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Default Schneider Symmar's

In article yPZfc.9888$432.4706@fed1read01,
"AArDvarK" wrote:

Now THAT is a good lesson, and a serious understanding, Bob!
His writing suggests it is genuinly impossible to create a true
apochromatic lens anyway, because of natural production
circumstances (for shorter lenses). I still do not have the
understanding tech-wise but some things are obvious. I still
would like to know what you know of the genuine APO quality
differences between Rodenstock and Schneider, if you know
of any, even if you are the distributer of Rodenstock. Are you
also the importer?

Alex


The distributor is always the importer. All distributors have contracts
with the factories they represent.

Both Rodenstock and Schneider manufacture to meet the DIN standards. As
for the difference between lens lines, well we both have literature and
published charts and graphs that are your for the asking. Even the
pre-publication version of the new digital lens brochure.

--
To reply no_ HPMarketing Corp.
 




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