Talk:Apochromat

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Old talk[edit]

This type of lens is properly called an 'apochromatic lens', not an 'apochromat'. The term apochromatic is much more widely used than apochromat, almost four times as often according to Google. We should consider renaming the article instead of redirecting from apochromatic to apochromat. Chris Jefferies 00:23, 4 January 2007 (UTC)
"Properly" is a matter of opinion, and probably depends on the field in which one works. The terms achromat and apochromat are well-established terms with a long history in optics. --Srleffler 03:37, 4 January 2007 (UTC)
You seem to have searched on the wrong terms. Apochromat gets 178,000 Google hits, vs. only 13,200 for the equivalent noun phrase "apochromatic lens". While the adjective apochromatic is more common than either, the page naming policy says that Wikipedia page names must be nouns not adjectives, except in rare cases. The Google search shows that apochromat is a better Wikipedia article title than the much less common noun phrase apochromatic lens.--Srleffler 03:50, 4 January 2007 (UTC)
OK, thanks for the interesting comments. I was a light microscopist for many years and never came across the term 'apochromat'. Maybe it's an American term (I'm European) or perhaps there have been changes since I left the field, but in the 1980s all the major manufacturers used the term 'apochromatic lens' or the abbreviation 'apo', but not 'apochromat'. Of course, it could also be that my memory is faulty :-( Chris Jefferies 15:15, 4 January 2007 (UTC)

Cost[edit]

So how much more does an apochromatic lens cost compared to an achromatic lens? The price must be the reason why all lenses are not apochromatic, right? —ZeroOne (talk / @) 18:00, 3 August 2007 (UTC)

Yes, they are much more expensive. Not sure exactly how much. --Srleffler 20:32, 3 August 2007 (UTC)

The exotic glasses used and IIRC occasional use of aspheric lens surfaces make for expensive materials and manufacturing techniques. Apochromats used to be outrageously expensive, though some are only wildly more expensive than achromats today. :-) Michael Daly 19:30, 23 August 2007 (UTC)

Details?[edit]

This only gives a general description of how do they work. Specifics would be good. 155.212.242.34 19:15, 14 November 2007 (UTC)

Commercial usage[edit]

Removed from article:

In commercial usage, for instance in photographic lenses specifications, "Apochromatic" might also mean the lens is in fact an achromat but one made with special low dispersion glasses. This can minimize the secondary spectrum to values typical of the first apochromats (late 19th century) in the market, made with the glasses then available, but technically not being an apochromat. Such lenses tend to be much cheaper than true apochromatic designs because they require less stringent tolerances during fabrication.[1]

This paragraph concerned me. While it does appear that Sigma is selling an "APO" lens that is not a true apochromat, the cite to their website is not sufficient to establish this usage of the term as valid. One might just as easily cite their website as a reference for a claim that dishonest manufacturers sometimes sell "APO" lenses that are not true apochromats. Neither claim should appear in the Wikipedia article unless it is supported by a reliable source.--Srleffler (talk) 04:11, 29 April 2009 (UTC)

Erm... ops, it was not my intention to attack Sigma or any other manufacturer in any way! In fact I own some of their "apochromatic" lenses and they are quite good... It's just that many lenses called "APO" or "Apochromatic", from many manufacturers, are in fact achromats using special glasses to minimize dispersion. You might also look at the first reference that states "It is frequently asserted in the rec.photo.* newsgroups that marketeers use the terms apochromatic and APO rather loosely." and also sites like http://starizona.com/acb/ccd/equipbasicsref.aspx that mention "semi-apochromatic" designs...
What is needed exactly to establish that many lenses advertised as apochromatic are not true apochromats, but the word is used to state in those cases achromat lenses using special glasses to minimize secondary spectra?
Rnbc (talk) 19:49, 29 April 2009 (UTC)
A better approach might be to have a section on semi-apochromatic lenses, that explains them and then mentions that some manufacturers call them apochromatic even though they are not true apochromats. It would be best to have one or two references to more neutral sources, rather than to lens manufacturers who have something to gain from spreading confusion over the meaning of "apo".--Srleffler (talk) 04:39, 30 April 2009 (UTC)
Maybe... well, I don't know what they have to gain since most top lens manufacturers also sell true apochromats and they call them APO just like they call the achromats with special glass. It's easy to find which one is which because true apochromats always have a triplet as a front lens group while achromats have a doublet. Rnbc (talk) 22:06, 2 May 2009 (UTC)
There is no need to invent a term "semi-apochromatic" when the existing term "achromatic" is more precise. --Nantonos (talk) 17:44, 25 October 2009 (UTC)
We shouldn't be inventing terms here at all, but can use terms that are in use in the real world. It's not clear to me that "achromatic" is more precise here, though. We appear to have two kinds of achromats on the market: traditional ones, and ones that have been enhanced to give performance closer to a traditional apochromat. Terminology is required, that distinguishes between these two types of lenses, and we need a way of talking about lenses that are marketed as "APO"s, but are not true apochromats.
That is easy, one needs a plot of focus position vs. wavelength. An achromat has two crossings of the axis while an apochromat has three. The Sigma and Rodenstock lenses marketed as "apochromats" and in fact achromats, and indeed the data sheet on the Rodenstock website clearly demonstrates this. --Nantonos (talk) 17:44, 25 October 2009 (UTC)
That's a great way for an individual to evaluate a lens he is thinking of purchasing. It doesn't help us here, though, because it is original research. Wikipedia is supposed to be based on reliable sources. To deal with this issue, we need objective sources that discuss these lenses. --Srleffler (talk) 00:30, 26 October 2009 (UTC)

Invention[edit]

Who invented the achromat, and in what year? CielProfond (talk) 03:32, 26 October 2010 (UTC)

This article is about apochromats. If you want to know about achromats, you should read the article on them instead.--Srleffler (talk) 04:18, 26 October 2010 (UTC)
Oops, sorry, that was a typo! I did mean to write apochromat. So here's my question again: Who invented the apochromat, and in what year? CielProfond (talk) 04:36, 26 October 2010 (UTC)

Concerns[edit]

I have many problems with the page. In my books on optics I can't find an optical engineering referece to this term. The examples used in the page are vague, do not specifiy wavelengths for the crossings and proximity, or what would differentiate optimal for visual vs. CCD imaging (typical). I add too that reference to Strehl and Airy values, how are they detemind? So the issue of wavelength in non-trivial, and lack standards of some metrology that can be verified. Polychromatic strehl based on what? Monochromatic based on what.

The glasses chosen should not be a part of the definition at all, as there are very many that could and are used - this has nothing to do with glass type and everything to do with wavelength and crossing in focus plane - I would remove any reference to 'glass type' used in lenses as the modern optical designer has many at his disposal that could be made to accomodate this term.

Without a solid foundation this is essentailly a marketing term, and should be stated as such. Does anyone have an internationally accepted engineering reference to this term?

Lastly, in amature telescope optics this term is applied very loosely and nearly always without documentation,(though there are minor notable exception)

One apparatus used to generate documnetation is found here

http://www.zygo.com/library/papers/proc_192_41.pdf

One web reference to the the topic of some value may be found here though it is in English

http://www.telescope-optics.net/refractor.htm

But again I do not see internationally acepted standards as a reference. Scientifically it needs necessary and sufficient terms to be a true difinition vs. an 'idea'. As is, this page is more of a concept than a true definition.

Thanks for looking, this is just my constructive criticism. what do you think?


DeepSpaceObject (talk) 02:57, 27 June 2012 (UTC)DeepSpaceObject

I can't address many of your concerns, but there is a clear distinction between an optical design that is optimized to eliminate chromatic aberration only at two wavelengths, vs one that is fully optimized at three. The former is an achromat, while the latter is an apochromat. The wavelengths chosen for optimization depend of course on the intended application for the lens. For an achromat, one chooses wavelengths at opposite ends of the intended band over which the lens will be used. For visible light imaging, one optimizes for a red wavelength and a blue one. I think 656.3 nm and 486.1 nm are the usual choices. I have seen infrared achromats optimized for a band in the IR instead. Obviously, a well-designed achromat will have low chromatic aberration for wavelengths between the two values for which the aberration is zero.
Obviously the next step one could make to improve on an achromat is to force the chromatic aberration to go to zero for some wavelength between the two ends of the band. That requires a more complicated optical design, and historically required less common optical materials. Glass type is at least of historical relevance. Classical designs required odd materials to get the chromatic aberration to go to zero at three points. I am not sure if modern designs actually manage to do this without using such materials or if lenses marketed as "apochromats" that do not use such materials actually only approximate the performance of a true apochromat.
I don't understand many of your questions, like the ones about Strehl and Airy values. What are you trying to ask?--Srleffler (talk) 05:51, 27 June 2012 (UTC)

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