Coma Example (Noctilux)

I'll have to post the detail shot oversize. Apologies to those with dial-up, but I kept the file size as small as possible. This is a detail from the upper left quadrant.

Note that over to the right (the center of the original picture) we have a bunch of bright highlights from small light bulbs. They are round, as they should be.

But at the left, we see, apparently, a swarm of butterflies moving in. But on closer inspection, they are not really butterflies. The lights approaching the frame edge are distorted into the shape of butterflies. This is coma.

The closer we get to the edge, the more pronounced is the butterfly shape. I rarely see this, even with the Noct at f/1. Just happened to catch it in this shot.

I have seen those oval highlights on many Noctilux samples posted here.

Not unexpected as mentioned in past technical reviews of this lens. If you want to practically eliminate coma, then get the 50/1.2 Noctilux with it's twin aspheric surfaces. To put it in pespective, the coma of the CV 40/1.4 is far worse than the 50/1 Noctilux.

I must say I have been unaware of this effect on my Noct. Now I will be looking for it! Does it only occur with specular highlights, or does the whole image get affected?

I must say it is still the best working tool at f1.0 despite the aberration. Maybe just accept the fact as part of the price for being able to shoot at f1.0.

Here I bought my Noct used for 400 bucks in the late 1970's. The tradeoff for a super speed lens like this is vignetting off axis versus abberations like coma. Vignetting is PURPOSELY added to a design to reduce abberations. In practice the F1 Noct lens here has very little flare, and has excellent wide open performance. It has a radically better performance than any of my F1.4 Nikkors in F mount, my Leica Canon 50mm F1.2

The older Noct F1.2 is today more of a collectors lens. There is one right now on Ebay at 5600 US dollars, the starting bid was 5500, it has 1 day + to go.

Doesn't look like coma. Coma should extend along a line between the point and the center of the image - and it should be Coma shaped!

Could be tangential astigmatism or possible (but less likely) out of focus highlights modified by the shape of the iris which is distorted by vignetting and/or oblique viewing near the edge of the frame.

Maybe ROB has weird shaped lights too! :)

Interesting picture. Can we really say this is coma?

First question: What is the shape of the tiny light bulbs? Are they round or are they pear shaped, sort of like the older larger Christmas lights? If pear shaped, some of the lights in the upper left appear pear shaped. However, for many the shape is clearly distorted no matter what they started out as.

Second: Coma makes a commet shape out of a point source. It would take small round lights and make commets with a dull (rather than pointy) front end. Also, due to the mathematics of third order coma, it would appear as having the brightest and smallest feature pointed towards the center of the field (the optical axis) and the outer edges away from the center would get wider and fainter. The butterflies in this picture do not behave this way.

The butterflies appear to be elongated tangentially rather than radially. This is a feature of astigmatism (different planes of focus for the tangential VS sagital rays).

I could not see any clear signs of coma but Rob's image shows definite signs of astigmatism.

ROB has bulbs that look abit like some coma, blended with Astigmatism. With a color shot the astigmatism would show up as some green and red; sort of a nice seasonal look for the house?

Not really Leica-stuff, but I did a test last year with a Noct-Nikkor. Perhaps they are useful here. Both shot were taken wide-open (f1.8 and f1.2 respectively). BTW, the Noct-Nikkor is well-known for coma correction.

>Doesn't look like coma. Coma should extend along a line between the point and the center of the image

What you are describing is astigmatism, not coma. http://www.vanwalree.com/optics/astigmatism.html

Point light sources rendered into seagull-shapes is caused by coma, though I've seen someone describe it as "sagittal oblique spherical aberration." Look at the coma-free Noct Nikkor vs. the plain-jane 50/1.2: http://www.nikon.co.jp/main/eng/portfolio/about/history/nikkor/n16_e.htm

Rob, post one of those Noctilux pics with swirly bokeh and see what people are gonna it. ;)

Alexander: You have interpreted the material in the link you posted incorrectly and also missed the critical shape of the light spots near the edge of the field in the image Rob posted.

Figure 7 in your link (http://www.vanwalree.com/optics/astigmatism.html) probably shows the sagittal focus of an astigmatic imaging system although it is difficult to tell without looking inside and outside of focus. The spots seen in Figure 7 in that link are elongated but not cone shaped. Cone shaped spots with the "point" pointing towards the center of the field would be from coma and they would persist both inside and outside of focus (slightly). Astigmatic spots would change from being elongated radially to being elongated tangentially if you shifted focus. From the one image in Figure 7 of your link, it is hard to tell and the image could have both astigmatism and coma.

Robs image is different. His spots are elongated tangentially which is a clear indication of astigmatism. The light sources in Rob's image show tangential focus and sagittal defocus - coma will not produce this effect.

Clear explanations and better examples can be found in the references cited in that link.

[1] Eugene Hecht, Optics, 3rd ed., Addison Wesley, 1998. [2] Born and Wolf, Principles of Optics, 7th ed., Cambridge University Press, 1999. [3] A.E. Conrady, Applied optics and optical design, part one, Dover Publications, 1985.

Or go here (http://aberrator.astronomy.net/index.html) and you can download a package which demonstrates common aberrations.

A good thread.

Bob Atkins, thanks for pointing it in the right direction!

Hmm, looks like I really started something here! I based my belief that I'm illustrating coma, on an illustration In "Advanced Photo School," Gunter Osterloh, 2005, p. 104. The formations in my posted picture look just like the ones Osterloh uses to illustrate coma.

I know that we have some formidable optical experts here--if my memory will serve, I believe Kelly is one of them. And I don't necessarily think that Osterloh knows it all. The responses to this thread are teaching me more about abberations.

BTW, this was not meant to be a criticism of the Noctilux. As I mentioned, I don't even see any problems in most of the shots I've gotten with it. Rather I was responding to an earlier thread in which coma was mentioned, and I though of my Noct shots as a way of illustrating coma. I had to search through negatives for a hour to find any distorted highlights, and then I only found one shot in six rolls that had any!

Keep the comments coming--they are very instructive!

ALL the worlds finest lenses will show abberations far off axis, when the lens is wide open, under harse nightime shots like these. The abberations are a mixture, usually a blend of several types. There will almost never be just one type, and none of all the rest. These simple examples are for college texts, and not complex blends.Coma and astigmatism

"Doesn't look like coma. Coma should extend along a line between the point and the center of the image - and it should be Coma shaped! Could be tangential astigmatism or possible (but less likely) out of focus highlights modified by the shape of the iris which is distorted by vignetting and/or oblique viewing near the edge of the frame."

Bob: You're right that the aberration is not coma. However, its not astigmatism either.

The aberration that causes these butterfly-shaped blobs is saggital oblique spherical aberration. Oblique spherical aberration is very similar to ordinary spherical aberration except that it is zero on-axis and increases as you move off axis. Tangential oblique spherical is normally absent due to vignetting at wide apertures. So you were on the right track with regard to vignetting. These are the classic aberrations of virtually all double Gauss lenses.

"ALL the worlds finest lenses will show abberations far off axis, when the lens is wide open, under harse nightime shots like these. The abberations are a mixture, usually a blend of several types. There will almost never be just one type, and none of all the rest. These simple examples are for college texts, and not complex blends.Coma and astigmatism"

A well-designed fast double-Gauss lens will have virtually zero coma or astigmatism at its design magnification. However, it will certainly have a ton of saggital oblique spherical aberration unless you use one or more aspherical surfaces to correct it. This is one case where the corner aberrations really are almost purely of one type and not a complex mixture. For many other lens types your comments are correct, however.

Now we have a real expert explaining :-) Thanks, Brian!

May-be I'm too simple-minded or just plain uninformed - but aren't we (as in any other wide-open shot of highlights from any other lens) just seeing projections of the shape of the diaphragm? Which in the case of the Noctilux wide open is a _huge_ circle at the heart of the barrel, necessarily intersecting with the circle at the front end of the barrel, to the effect that the more the highlights are off-center, the stronger that intersection will be...?!

Interesting perspective Lutz.

I will add couple of examples here of wide open shots with perfect circular apertured lenses. The OOF/highlights are discernable, I hope.

Here is delibearte out of focus shot through a Zeiss Biotar 75mm f/1.5 (wide open). This is not a symmetrical design.

This should be a real challenge for all optical experts! On axis, it is a superb (55mm f/1.2 Nikkor O, made for CRT copying
at 1/4 to 1/5.5 magnifications and with >200 lp/mm resolution)lens.

Mounted on a Bessa L, it can be made to focus to infinity.

A test result.

I know! I know! My turn! It's curvature of field. The lens was corrected for close distances, for CRT work. At longer distances, it doesn't have a flat field.

Am I right?

Rob, This lens was designed to have a negative field of curvature at its optimal magnification(s), so that the curved CRT screen would be rendered flat, on film.

Who could figure out all the off axis mush (that is how much technical I can be) projected by it at infinity? :-)

Cool stuff for Bob and Brian:

"Oblique spherical aberration"


"Definition:

A type of aberration in which a symmetrical light patch is formed at points that do not lie on the lens axis. When the image moves away from the center of the field, the patch increases in size, aiding in the formation of coma. Also known as abaxial spherical aberration."



@FIN

So, does this mean that after all that spherical abberation talk, it's coma anyhow?

Coma?

Hi Roland - and a happy New Year! Well, your's is a "schoolbook example" to illustrate my point. I recall wide-open close up shots made with fast Zeiss 1.4 primes for cinematography sporting the most peculiar triangle-shaped highlights, due to the pattern formed by the three blade diaphragms. A pity that I don't have stills from these readily available...

I don't think this thread is done with. I'll bookmark it, in case it pushed down the list.

Lutz, I have pentagonal highlights from some of the S-Planars. No scans on hand to show.

Just to clarify: The pentagonal highlights show only when the diaphram (pentagon shaped) are used. Wide open (circular aperture) they do not show.

This still does not explain the aperture playing a part in bringing out the oval highlights.

Just remembered the behaviour of some other S-Planars (highly corrected for most aberrations) with no iris diaphram. The 75/1.4 S-Planar does not project any butterflies. Only smooth circular highlights.

"Cool stuff for Bob and Brian:

"Oblique spherical aberration"

"Definition:

A type of aberration in which a symmetrical light patch is formed at points that do not lie on the lens axis. When the image moves away from the center of the field, the patch increases in size, aiding in the formation of coma. Also known as abaxial spherical aberration." "

Kelly: Some people will claim that any assymetrical off-axis aberration is "coma". Technically this is not true, and is just an oversimplification of what is really going on.

Of course, the distinction is really only important to lens designers, who really have to know what sorts of aberrations are present in order to correct them. I look at transverse ray aberration plots constantly in the course of doing design work, and there really is a big difference between coma and oblique spherical aberration, in terms of how the aberration arises, what effect it has on images, and how you fix it.

High order aberrations are not the sort of thing that can be settled or understood with a simple definition pulled out of cyberspace. Even excellent optical texts such as Warren Smith's "Modern Optical Engineering" only scratch the surface of this topic. Its the sort of thing you really only get comfortable with after working in the field for a decade or more.

The FAR edge image quality at F1 was called "a mixture of skew-ray flare and coma" in March 1976 in Modern Photgraphy's Test report. Skew-ray flare is not used that much in naming abberations. Type skew-ray flare in google and you dont really get 24 hits. Place it in quotes and there are only 2 hits on a Olympus OM site.

Each lens element usable diameter becomes a possible "stop" in ray tracing. These cut off off axis rays of a fast lens alot. One can purposely fiddle with these diameters on a design when tweaking the abberations down. They are effectively a "stop". Vignetting is purposely added to reduce/control far off axis abberations. ; :) Plus nobody is going to afford a lens so big that all the viewfinder is blocked, that costs somwhere between a car and a house. :)

Vivek, a) I stand partly corrected: the triangular Zeiss highlights would of course show up only with the three-blade diaphragm at least slightly stopped down, b) this observed effect plus the ones you mention just confirm the point, that OOF highlights take the shape of the diaphragm aperture. Because the aperture in a Noctilux will intersect with the barrel front end (starting quickly from angles off-center) the resulting projections will take the shape of not full but decreasing(?) moons, with their orientation towards the center of the image. If you have a Nocti at hand, try to look through it against a bright source of light, wide open. Turning it sideways will soon produce the effect described. If you stop down at a given angle, the aperture will eventually be completely visible again (because now it's smaller) and no longer be intersecting with the barrel's front end. From that f-stop down, at the given angle, the "decreasing moon effect" should no longer be visible.

Lutz, Honestly, I have to accept (though it is way over my comprehension) what Brian Caldwell said. It was interesting to see the perspectives from users. The Biotar example I showed has the same charcteristics (OOF highlights) of the Noctilux 50/1. The 50/1.2 Nocti lacks that (check the thread on Nocti shots and also the examples from Regit, above).

It is not a simple matter of intersecting apertures.

However, in the final analysis, it does not matter. I am reminded of the workings of a lamp. With a bit of education, I could understand how a candle works, later how an incandescent electric bulb worked or a flash bulb. When it gets a bit too complicated with micrwave induced light bulbs and the like, it is back to square one.

Plug it in, switch it on and get the light.

Thanks again to Brian for his insights.

Fabulous thread!

Nocti Thread: http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Ejjq&tag=

Ciao Lutz! Dir auch ein schoenes Neues! Waere wieder mal Zeit fuer ein Treffen. Was denkst Du?