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#32
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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
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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
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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
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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
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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 |
#37
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Schneider Symmar's
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#38
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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
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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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