28mm test:CONTAX 28 f2, NIKON 28 f1.4, YASHICA 28 f2.8,CANON

Orio · Expire: 2012-12-04

Coma is a Latin word which corresponds to today's italian "chioma" (english is something like "flowing hair" or "mane").
"Cometa" (English: "comet") is the diminutive, it means "piccola chioma" ("small mane"), because that is what comets look like when you see them from the Earth.
The meaning of the word is a perfect explanation for the appearance of the optical aberration called coma: light is deviated to form a trail that extends radially from the source.
The fact that is an aberration of transversal type means that it's not altered by different focusing, because it always lies on the focusing plane.
It's appearance can instead be progressively reduced by stopping down the iris of the lens.
You will notice that it can be altered the same way as purple fringing (transversal chromatic aberration), that's because both are aberrations of transversal kind.
Astigmatism is also a spherical aberration of transversal type. In spherical lenses it mostly appears as oblique astigmatism, i.e. the result of sagittal and tangential light rays focusing on different points of focusing plane.
The main visible difference between a coma aberration and an oblique astigmatism aberration is that coma produces blurring, whereas oblique astigmatism produces distortion. This of course is very roughly said (not scientifical description).
Many people who notice distorted out of focus highlights at the edge of an image think it's coma, while it's oblique astigmatism instead.

Coma is much more annoying because it creates radial blurring which is not only noticeable in out of focus highlights but impacts also an image in perfect focus.
Residual coma aberration can be noticed at the edges of very fast lenses of spherical type used wide open, such as the f/1.2 Nikkor in Rockwell's example. That is absolutely normal and quite unavoidable.
In all other lenses (99% of lenses) it should not be noticeable if the lens is of good quality, because making sure that coma is corrected is a top priority for lens makers.
Much more common (in spherical lenses) are astigmatic aberrations, for two reasons: 1) they are much harder to eliminate (without the help of aspherical elements) and 2) some manufacturers
choose to not correct them completely, because they can add to the character of a lens.
I remember having read the statement of a Zeiss engineer who openly admitted that field curvature (perhaps the most important consequence of astigmatism) was NOT entirely corrected ON PURPOSE
in the new Z-series 2/28 Distagon because it contributed in a decisive way to the feel of images created with the original Contax "Hollywood" lens and that Zeiss buyers expected to find again in the new Z version of the lens.
Like it happens with humans, a lens with some characteristic imperfections is more appealing than an absolutely perfect one.
For instance, many males prefer an "imperfect" (for classical canon) female like Brigitte Bardot to an absolutely perfect statuary body like Bo Derek Wink

The link that I gave in my precious message:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/coma.html
contains text explanations that are very clear if you take the time to read them.
Other useful readings are:
http://www.aristidetorrelli.it/Articoli/Ottica/Parte3/Aberrazioni.html (Italian Language)
http://toothwalker.org/optics/astigmatism.html (English Language)

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

Thanks Orio for the details.

I posted two pictures that, following what you wrote, should explain the difference:

this should be Coma

this one should be astigmatism

is it?
I selected ,most probably, the two best lenses to show it

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

If I understand well, first image has a small amount of coma, but huge amount of both trasversal and sagittal astrigmatism. Second has a bit larger amount of coma, but a very small amount of sagittal astigmatism and modest transversal astigmatism Smile

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

It should be exactly the opposite.

Orio · Expire: 2012-12-04

Both images show both coma and astigmatism (and also transversal chromatic aberration, and possibly even some axial chromatic aberration too - are they corner crops?).

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

Can I note on the photos what I see as coma and astigmatism?

Orio · Expire: 2012-12-04

When the bright point seems to have tails, that is coma. Easily identifyable in the images posted by malsano.
When it looks like oval shape, that is oblique astigmatism.
For an example of oblique astigmatism, I borrow this image from Robert T. Wilson, that he posted somewhere else on this forum.
You can see the effect of oblique astigmatism in the shape of the highlights at the edges:

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

I give up...more I read on webpages, more I only see only astigmatism and no coma here Smile

Aanything · Expire: 2014-05-30

So the "swirl" some lenses show in the oof highlights has something to do with this kind of astigmatism?

Orio · Expire: 2012-12-04

koji · Expire: 2012-12-27

Here is typical coma of a wide angle lens, focused to the farthest tree.

These coma are called "front coma", i.e. coma appears in front of focused plane.

There is "back coma" too, generally speaking "front coma" is worse but
for some lens "back coma" is worse (I do not have an example in hand).

Orio · Expire: 2012-12-04

Thanks Koji, excellent example.

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

The two posted exemplum are, as told, selected because are exactly the opposite in terms of projects.

The first one Is an Olympus OM Zuiko 50 1,2 used at 1,2 on canon 5d mk III
a good "old" project without aspherical element. It should be the classic lens that can generate Coma

a good "old" project without aspherical element. It should be the classic lens that can generate Coma

The second one is a "modern" project with TWO aspherical elements. A lens that should be able, thanks to the aspherical element (I selected this one even because it has two asph..) , to avoid coma, but may be afflict to astigmatism following the kind Orio's information.

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

This is a Canon 50L 1.2 @ 1,2

as expected is different respect the Olympus ( I think "butterfly lights thanks astigmatism and COMA as the aura around the light ) and similar to the Canon 50 1.0 L in terms of lights results in the night.

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

to complete the matter...

Contax 50 1,7 AE at 1,7
The coma is present but the astigmatism is very well corrected

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

we e totally out of topic Cool

is it possible move this part from 28mm test and create another topic " Coma and Astigmatism " ???

thanks

sichko · Joined: 20 Jun 2008 Posts: 2475 Location: South West UK

Excalibur · Expire: 2014-04-21

Very interesting and have learnt something as I've just accepted lenses which probably had coma and Astigmatism and just thought "that's what you have to put up with, when using camera lenses" Rolling Eyes

malsano · Joined: 27 Nov 2012 Posts: 93 Location: Dubai

yes! cropped.

So at the end even modern lenses have coma Smile