Lenses with bad/strong Coma?

[former member]

The term "comatic aberration" is not found in any of the formal books on optics that I've got. The term used is "coma", and it is listed as a lens aberration.

I suppose it, like the work "Bokeh", will be found in every-day forum language from now on. It is descriptive of the optical property, as is Bokeh.

Astigmatism is also responsible for the football-shaped "swirlies" in lenses, including the Summarit.

 

[rxmd]

The term comatic aberration, strictly speaking is incorrect in English but is creeping into common use (which means it will soon become correct usage, in all likelihood), probably from German.

I don't think so, coma in German is Koma while the other aberrations are called aberrations. On Google you find very few hits for "comatic aberration" in German (about 5) and a number in the thousands for English.

Philipp

 

[former member]

The term "comatic aberration", from a quick google search, appears to be used in formal papers and patent applications going back decades. It is more exact than "coma" which has meaning outside of optical design. In books on optical design, "coma" is within context and is the term used. In patents, I can see the expanded term used as definitions must be exact in scope. I've gone through the patent process within the field of optical communications, and the exact meaning of words is "very binding". Never use a two-cent word when a fifty cent word will do.

 

[former member]

Okay, so Roger got me spun-up. I'm not used to seeing the term "Comatic Aberration" in text books on optics and optical design. I went through a couple here at work. "Coma" appears to be the much more used term in engineering books. "Principles of Optics", Max Born and Emil Wolf, 1959 uses "Coma" but "Comatic Aberration" does not appear. "Optics" by Hecht and Zajac, 1979, uses "Coma" in the index. The text uses "Coma, or Comatic Aberration," ...

So this is one instance where the Physicists and Engineers chose the simpler term for technical writing, rather than a much more exact term. That is fairly rare. The term "Bokeh" does not appear in any of these text books...

 

[kiev4]

This article on monocle lenses:

http://kiev4.narod.ru/tutorial/kiev4_02_e.html

Posted by a new RFF member has several examples of images with strong coma, but not "deep Coma".

Very interesting to me what are "strong coma" and "deep coma"? 🙂 As for me I see on your example photos not only coma but great curvature of field. And on my examples the curvature of field is not so great. That's the only difference, I think. Maybe I'm mistaken.

 

[former member]

Deep Coma is what I feel like at the end of the work-day! Kind of like that photo...

 

[principe azul]

On coma and comatic aberration, my guess is that it's down to confusion - in popuar usage - with *chromatic* aberration, which seems to have become a big talking point once people started using lenses designed for film cameras on digital cameras.

 

[kiev4]

Deep Coma is what I feel like at the end of the work-day! Kind of like that photo...

🙂 Good joke. So do I.

As far as your image is concerned it seems to me that the effect is not produced by coma. You can easily check this by moving the diaphram here and there. If it is coma - the images blurring at the edges will be different. But I think this is curvature of field (I do not know if this is correct translation of the term from Russian). I can explain what I mean. The meniscus concentrates rays not in focal plane but in a "curved surface" like plain with bended corners or like sphere. This results in images like your example because the film sirface is flat and farther from the center of the picture - more discrepansy between the film surface and the "focal surface". This effect does not depend on the diaphragm position, it depends of aperture.

 

[former member]

> But I think this is curvature of field (I do not know if this is correct translation of the term from Russian).

Your translation, and English, is excellent. My thinking on curvature of field: I would have expected "something" in the image to be in focus as a function of distance from the center. As you look at the image from edge to edge, everything goes out of focus quickly. If "only" curvature of field was involved, something should have been in focus, either closer or farther from the main focus at the center. This lens seems to capture all of the aberrations at once.

 

[Cale Arthur]

On coma and comatic aberration, my guess is that it's down to confusion - in popuar usage - with *chromatic* aberration, which seems to have become a big talking point once people started using lenses designed for film cameras on digital cameras.

I thought chromatic abberations only occured at open-mic nights and guitar stores? 🙂 ..sorry, couldn't resist.

--c--

 

[kiev4]

This lens seems to capture all of the aberrations at once.

I agree with you: both coma and curvature. And my observations shown that the more lens diameter - the more curvature of field. My 42mm monocle with 20 mm lins diameter has less curvature than 135mm one with 40 mm lens diameter. And the coma is minimized for both of them,